Daphnia or "water fleas" are incredibly tiny freshwater crustaceans familiar to anyone who's taken a biology class, which happens to be the vast majority of Japan - explaining why they already crop up in anime and video games. Their single compound eye gives them a cycloptic appearance, and they're known for their rapid reproduction.
Pokétential: this might be an extremely small and rather weak pokémon, but could get an automatic stat boost each turn as it multiplies into a larger swarm.
This famous but poorly understood creature was an apex predator long before any dinosaur roamed the Earth, and was a closer cousin to modern mammals than to reptiles. The huge "sail" is believed to have helped absorb heat from sunlight.
Pokétential: obviously, something that benefits from the game's sunshine effect. This would usually mean a grass or fire type, either of which have great design potential, but I'd prefer something less expected, like a solar-powered rock type with a crystalline fin.
Sharing the permian period with Dimetrodon, (and coming right after it due to my alphabetic listing. Convenient!) this large salamander is known for its strange boomerang-shaped skull, the purpose of which is totally unknown. It could have been used to glide quickly through the water, root for prey in mud or make the animal a more difficult mouthful for predators.
Pokétential: for whatever reason, I look at this guy and see an electric type. Maybe its head could be a giant magnet?
Fans have long clamored for a simple bottlenose dolphin, but those overhyped rapists and murderers aren't nearly as cool as the Amazon river's eerie fresh-water species, with their nearly vestigial eyes, crocodilian looking jaws and dark mythological association.
Pokétential: water/dark would honestly suit a river dolphin or even a bottlenose, what with the latter's often ruthless gang-bang hunting tactics. I might even like a bottlenose dolphin poke' if it were some pugnacious brute with jutting teeth and battle scars.
Rolling around huge balls of excrement as food for their offspring, dung beetles do an excellent job keeping the environment a little bit cleaner. The ancient egyptians found them especially inspirational, with stories of a great beetle rolling the sun itself.
Pokétential: drawing from nature, we would have one stinky bug/poison type. Drawing from mythology, a fairly awesome bug/fire. Perhaps its type could vary, with additional options like a snowball or a boulder?
This terrifying ancient sea monster could exceed thirty feet in length, with a thickly armored face and a nightmarish beak formed from guillotine-like plates of bone. Able to chomp most other animals clean in half, it was one of the largest and deadliest marine predators the world has ever seen.
Pokétential: pokémon has a long tradition of prehistoric water/rock types resurrected from fossils, and I can't see any excuse for Dunkleosteus to break that tradition, possibly even as an evolution to our Coelacanth pokémon.
These under appreciated and unfairly maligned insects are harmless to humans, but possess powerful pincer-like appendages on their tails used for a variety of purposes, especially to fight off rivals. Unlike many insects, females take tender care of their babies. Most species also possess rather beautiful wings, but very rarely use them, folding them tightly beneath tiny shields on the thorax.
Pokétential: an earwig is as good a choice as any for one of those early game bug types, though I'd rather see something with a little more bite to it; maybe a bug/fighting type with scissor-like tail blades.
Personal favorites of mine, these irritating but fascinating bloodsuckers may be one of the world's most famous parasites, known for their small size, itchy bites and nearly unmatched jumping ability.
Pokétential: there's a lot of fun to be had here. I'm imagining an extremely tiny bug type with the stats to match, but high evasiveness and devastating status moves. Perhaps it could show up in your party automatically after battling some mangy, filthy dog pokémon?
My personal favorite animal group, the Diptera are amazingly diverse and widely beneficial, though their most famous representatives are best known for living in and eating all the most malodorous, unmentionable filth known to man - right after they throw up on it!
Pokétential: out of every single thing on this list, I cannot believe there isn't already a proper Dipteran pokémon. I'd be thrilled if its design were another Japanese take on Beelzebub.
Related to pelicans, frigate birds are also known as Man of War birds and even pirate birds for their occasional tendency to steal food or even raid the nests of other birds. Males are distinguished by a brilliant red pouch they can inflate to catch the attention of females, not entirely unlike that of a frog.
Pokétential: it's surprising how few pokémon have an even accidentally piratical theme, so a thieving black and red seabird seems as good a point as any to start. It might have jolly roger markings on its wings or throat pouch, and an added head crest like a captain's hat.