MONSTRO:

The most basic boss is just a really huge, gluttonous head with bad teeth and a cleft lip, which attacks by crying and puking blood. While the simple stylization of the game's monsters leaves this open to interpretation, it's generally assumed that they're all monstrously altered children, possibly even Isaac's siblings or reflections of Isaac himself. We're looking at a gigantic, malformed baby head that bounces around crying in a perpetual rage. And that's beautiful.

LARRY JR:

Like Monstro, Larry junior is simple enough to be kind of a poster child for the game's monsters. He's your classic segmented, serpentine boss monster, ala Moldorm from the Zelda series. This horrible baby-worm-snake just sort of slithers and chomps along in random directions, making no real effort to come after its prey but leaving a nice disgusting present to step in every so often.

GEMINI:

A little fetus attached to a bigger fetus by their shared umbilical cord. Does that make them siblings, or is one the child of the other? They obviously love each other very much, but like every other monster, it's you or them I guess. The little guy will detach and fly around when the bigger one takes enough damage.

STEVEN:

A variation on the Gemini boss, Steven is also a cameo from one of Edmund's other games, but I like its jarringly different art style better in the context of this one, inexplicable and otherworldly compared to the other bosses. More of a shadow-being than a flesh and blood horror.

THE BLIGHTED OVUM:

There are several "post-mortem" versions of various bosses, and Blighted Ovum is the vengeful cadaver version of Gemini; the older brother already a corpse, the younger brother a drifting ghost. At least we know dying doesn't keep the poor little guys apart.

PIN:

A giant pinworm with what may be a child's face is by far one of the worst monsters I can think of if you're like me and easily nauseated by other people's hygiene. I live in fear of ever experiencing pinworms, as darling as they are under a microscope, and it's not even because of the parasites so much as the fact that if I get pinworms, it means something from a stranger's anus got into my mouth somehow. That's how they spread. Fecal residue. Especially the fecal residue of children, since they're less inclined to care if they scratch their own ass and smear it on a shopping cart. HAPPY HOLIDAYS!

WIDOW:

Even the "spider" boss is just composed of mangled-up child anatomy. I especially like the huge toes on the ends of its fleshless legs. Small-size spider enemies also demonstrate infant-like faces, though it's hard to say if that applies to the still tinier-size spiders. I'm going to say yes. Do you suppose even all the tiny flies and maggots might have baby features, up close?

THE HAUNT:

A new boss added to Rebirth, the Haunt isn't as strange as most of its brethren...kind of just like any other big, round ghost, really. Like a Boo Buddy! Well, I guess they just call them "Boos" now, but when Super Mario 3 was brand new? They were Boo Buddies, dang it. Sometimes, the haunt's sadder ghosty face will peel away to reveal a giant skull, which is especially lovely under the "everything is a baby face" hypothesis.

DINGLE:

Hilariously, one of the new features in Rebirth is just the addition of a whole lot more shit. Especially in the form of more shit monsters and a whole new shit boss. The deliriously happy-looking and repulsively named Dingle can summon lesser shitmonsters by whistling (a brown note?), and very alarmingly uses blood as its go-to projectile weapon. You might brush this off as toilet humor, but I always found the notion of shit monsters sincerely horrifying, which is also a form of humor, but also a form of horrifying. The goofy cheerfulness of Dingle only makes it all the worse.

THE DUKE OF FLIES:

A fan favorite, the Duke is just a swollen, eyeless corpse who drifts around without a care in the world - and doesn't even have much health. He might not even be all that dangerous if not for the flies constantly swarming out of his dessicated, hollow body. Who even cut him open and stitched him up? I appreciate how much everybody seems to love this guy's laid-back personality and adorable pudginess, despite being a big reeking balloon-fetus full of flies.

FAMINE:

The horseman boss of the first chapter, Famine adorably rides one of those stick-horse "toys" with an actual, rotting horse head impaled on it. The dry, cracked skin is a nice touch, too.

GURDY:

Another fan favorite, Gurdy is a nasty little girl who pops in and out of her own tumorous outer body, popping flies and giant boil-creatures out of her various tubes and pores. She seems to have a grand old time of it, too, with a big grin on her face whenever she tunnels back out of her own blighted flesh to barf in your general direction. Gurdy has life all figured out.

GURDY JR:

Gurdy was even so popular, subsequent expansions have decided to keep giving her an extended family. Gurdy Jr. was added in the Wrath of the Lamb expansion, and unlike Gurdy, Junior can actually move around. Amazingly fast, too, for a baby imprisoned in its own externalized polyp. Apparently that stuff bounces like silly putty.

GURGLINGS:

This even newer member of the Gurdy clan might count among the game's most adorable monsters. Look how much fun they're having, just like their eldest sister! I especially enjoy the fact that they don't appear to have complete bodies under there; just heads, nubby arms and possibly part of a torso concealed under the malignant flesh.

FISTULA:

NI always really liked monsters that consist of just a faceless glob or lump; especially when they float. There's just something extra creepy about a big clump of flesh drifting around the room. As you battle Fistula, it will keep splitting into smaller and smaller chunks, each of which finally leaves behind a maggot!

BLASTOCYST:

Good old Blastocyst, just an embryo in a giant lump of jelly. Like Isaac, its primary method of attack is to cry at you, possibly out of envy towards children who've already been born. Or maybe the gelatinous cell is the real monster, and the embryo is just kind of along for the ride, like a goldfish?

CHUB:

You can't easily tell from her art, but Chub is a gigantic, bloated maggot. Apparently this isn't the kind of maggot that matures into a fly, since she's constantly giving birth to less-giant maggots. I'm assuming Chub, too, really has a face a lot like Isaac's.

C.H.A.D:

Another variant on Chub, this is also a cameo boss from Edmund's Super Meat Boy. The more you damage this meaty grub, the smaller and faster he gets!

MEGA MAW:

This new boss is a giant version of the "maw" enemy (shown above), though I have to say, the tiny version is a hell of a lot more frightening looking. That expressionless face with its hollow, gaping mouth and bleeding eye sockets is a lot worse than Mega Maw's scowl. Still, a bleeding giant head is a bleeding giant head. Nobody turns down a bleeding giant head.

MEGA FATTY:

Another new boss in Rebirth, this guy barfs, bleeds, farts and defecates as offensive mechanisms, virtually every single thing you don't want a corpulent monster baby or a regular baby to do when you're locked in the same room.

THE HOLLOW:

Another of the "post-mortem" monsters, Hollow is the dead version of Larry Jr, and if split apart by your attacks, its sections will sprout their own new heads. I love the longer, thinner teeth in its main artwork, it has such a nice, haunting, deep-sea vibe.

THE HUSK:

An even more decrepit, less jolly version of the Duke of Flies, sans even his cute, vestigial limbs! He's little more than skin stretched over a skull, and he'll even spit up spiders in addition to his flies. I like that. It's like being filled with flies is normal and healthy for the Duke, so of course the dead version is going to have some spiders feasting on his precious flies.

PEEP:

I like how peep here is only semi-human in his anatomy, somewhere between a child and a big fat grub. Unsettlingly, his first method of attack is to just keep pissing himself, until later in the fight when his eyeballs pop out and fly around the room. There's something endlessly funny about a monster who won't stop urinating, and then his eyes fly out. It's like some beautiful setup and punchline.

THE WRETCHED:

There's not a lot to say about the dead version of Widow, except, oddly enough, its head seems to have been turned right-side-down following its death.

THE CARRION QUEEN:

Post-mortem Chub lends more credit to the idea that she had quasi-human anatomy; why else would a maggot have had a skeleton to reanimate? The best part of this skull-maggot, or skuggot, as we in fake science like to call them, is that she craps deadly turd piles that grow back again and again, one of the worst things a turd pile could ever possibly do.

POLYCEPHALUS:

A new boss in Rebirth, this nightmarish being is either a multi-headed monster with a huge face for a torso, or simply keeps its lower body buried, exposing only a gigantic head laden with parasitic siblings. Really picture that. Really think about a giant baby tunneling out of the floor with more babies hanging off of it like giant warts, bleeding profusely from their gouged-out eye sockets.

DARK ONE:

I usually prefer more ambiguous phenomena than just your plain old "demonic forces," but I've come to reallyy appreciate the Biblical horror side of this game I never personally played. As the abused child of a deranged religious zealot, Isaac may very well have been instilled with a terror of being sinful, evil or unholy himself, making this is an especially chilling boss encounter if you like that "every monster looks like Isaac" theory.

PESTILENCE:

The second horseman boss, Pestilence naturally attacks with flies, which is more or less what "pestilence" most properly means. The dictionary these days just defines it as "diseased," but it's supposed to specify infestation by insects and parasites.

THE CAGE:

The dead version of Mega Fatty is far, far more terrifying for simply lacking a head. There's something about a giant, blobby torso with no head or face that's just so much more upsetting than either a non-blobby torso with no head or a blobby torso with a head.

MONSTRO II:

Dead Monstro is almost entirely bone, with just a scrap of flesh left on its face. We also get to see its dangling, vestigial skeleton here, so now we know Monstro wasn't just a severed head, but a monster whose body degenerated, which is an important distinction.

THE ADVERSARY:

Dead Dark One, missing a horn and wrapped in bandages. Not the most inventive of the dead bosses, no.

GISH:

Gish was once the title hero and player character of his own side-scrolling game, before Isaac was a twinkle in Mcmillen's eye. The sentient blob of tar went up against a slew of monsters not at all unlike those encountered in Isaac to rescue his goth, human girlfriend, and now, for whatever reason, he's taken to slinking around basements and murdering children. What happened, Gish? Is this how you dealt with getting dumped? Wait.... did your girlfriend become Isaac's mom? Is that what happened?? ARE YOU ISAAC'S REAL DAD GISH

THE BLOAT:

I'm not really sure if Dead Peep is creepier than regular Peep. On one hand, it has a grosser face, it bleeds instead of just urinating and its eyeballs are always flying around, but with regular Peep, you've got that contrast between its disgusting behavior and comical appearance. Plus, the flying eyeball thing gets to be a surprise! Like mine!

DADDY LONGLEGS:

An even bigger spider monster, so massive that its body dangles from above while its legs attempt to smash you.The dangling tongue and lack of a lower jaw really set this design apart from your ordinary head-spiders.

MASK OF INFAMY:

One of my favorites from my 2012 article, this monster consists of both a hovering, bleeding, giant heart and a hovering, bleeding giant "mask" that appears to be more of a huge fleshy head, frozen in anguish. What's especially interesting is the thing peeking out of the mask, which is either a huge eyeball or a creature with a beady black eye...it's difficult to tell in the art style. According to Mcmillen, the thing inside "is not alive."

TRIACHNID:

A variant on Daddy Longlegs with only one eye and three appendages, this is actually another creature from one of Edmund's other games. I'm going to pretend this made all of them canon and almost all of his monsters came out of the same horrifying mom.

THE GATE:

This thing is possibly a giant version of Hosts, monsters consisting of just piles of organs and parasites topped by skulls. The great thing about Hosts is that they, too, were once stars in their own game.The great thing about the Gate is that there's apparently another Isaac controlling it from inside.

LOKI:

Little to say about this one, really; Loki is a devilish child with a triple 6 on his forehead. His first appearance is as a boss, but he quickly becomes a regular enemy.

WAR:

The third horseman is suitably grouchy looking, and appropriately attacks with bombs.

MOM:

You eventually battle what is either your own mother or a corrupted manifestation of her, her appendages and even eyes bursting into the room from the walls and ceiling to murder her own son. About the only thing more horrifying is the fact that later stages take place in your mom.

MR. FRED:

One of the bosses unique to the interior of your mother, Mr. Fred looks a lot like a monstrous embryo, but that circle of teeth is distinctly less human. That might explain how he's seemingly able to just tunnel freely throughout your mom's body. I especially like the globs of gelatinous flesh he has for eyeballs, definitely one of my favorite designs in the game.

THE FALLEN:

Another demon, and not really as creepy looking as The Dark One. I can't say I'm all that impressed. It's another enemy that splits into two smaller forms as it takes damage.

SCOLEX:

Named after the anchoring appendage of a tapeworm, Scolex gives more of a hookworm impression if you ask me, but also seems like another "child" half-formed into a very different organism.

TERATOMA:

Possibly my favorite boss, Teratoma has everything I like about Fistula - an ominous, indifferent clot of drifting tissue - but with lovely hairs, teeth and blackened flesh. Plus, it's filled with spiders. I like flying polyps full of maggots as much as the next person, but there's something special about this withered knot of organic chaos being filled with spindly, fuzzy arachnids. It really fits, somehow.

MAMA GURDY:

In Rebirth, it turns out that the gurdy we all knew and loved wasn't even her final form. In your Mom's body, you'll find yourself in a whole boss chamber that seems to be a Gurdy tumor-body, Mama Gurdy's massive arms and head emerging to battle. Are the Gurdies just those tumors that grow tiny baby limbs in them, or something? That happens, you know.

DEATH

The fourth horseman is, of course, mostly skeleton, and armed with orbiting scythes! I love the art of him riding his horse more like a surfboard, too. Of course he would.

CONQUEST:

An alternative to the Death boss, "conquest" is actually the horsemen pop culture eventually replaced with the bug-loving "pestilence." I know which one I prefer.

LOKII:

Dead Loki is a lot more fun than regular Loki. Who doesn't love a flying baby that can split itself in half? Or any baby that can split itself in half, so you don't have to do it for them? What a well behaved child.

IT LIVES:

I'm kind of skipping a boss here, since you originally fight your mother's heart at this point, but in subsequent battles, the heart will transform into a heart-fetus creature ominously named IT LIVES. There's really no telling the story here. Your mom has a baby for a heart? A baby became her heart? Is it you? Is this just an Isaac?

SATAN:

It's hard to take the classical depiction of the devil very seriously. Goats are like the silliest animal that exists. I suppose it's all the spookier for the manifestation of all evil, hate and deceit to have a ridiculous animal head, but I dunno, I'd have gone with a pig. Pig heads are way scarier out of context than a goat head out of context.

ISAAC:

Naturally, Isaac will eventually have to fight himself in the most literal and straightforward sense, lending further credit to the idea that he's been technically doing that all along.

???:

Better yet, another, unnamed boss is really just a dead Isaac. That's it. There's no need to make any more jokes about anything again, we've already reached the absolute peak of all comedy in forcing a toddler to fight its own corpse.

THE LAMB:

Of all the demon bosses, this is probably the coolest looking and creepiest. Lambs are definitely less hilarious and slightly spookier than goats, and I love this thing's face. This is another of those Isaac monsters where you can kill either just its head or just its body before the other, but the fight still won't end. No matter how many video game creatures pull that trick, it never gets old for me. How I envy them.

MEGA SATAN:

Enh. Again, not as impressive as any of the non-demon bosses, and not as eerie as The Lamb. Mega Satan is more minotaur-like than anything else, really; he doesn't even quite fit the game's style. Ah well.

THE HEADLESS HORSEMAN:

The last bonus horseman isn't even the Apocalypse kind, but the Sleepy Hollow kind, which is perfectly okay. I like how much fun he's having with his own head.

KRAMPUS:

I really do enjoy the increasing popularity of Krampus these days. If he grows as popular as Sandy Claws in my lifetime, I may even start to get a real kick out of Decemberment's Evesday. Krampus is an unlockable bonus boss you can fight throughout the game, sometimes replacing the "Devil Rooms" you normally use to acquire items. Evil items.

ANGEL:

I'm glad more people are also taking advantage of the fact that angels are such disturbing monsters in their own right, though I'm surprised the angel in Binding of Isaac isn't a little stranger. Biblical angels had like a million eyes and multiple animal heads. I guess it's fairly ghastly that it cries blood tears without a face, but you know, it might be cool if another Isaac expansion added a new slew of bizarre "angelic" monsters.