After the "big guys" of a game's legendaries, we usually get a callback to Mewtwo and finally a callback to Mew, though we already saw the latter back at the beginning of this generation when we reviewed Victini.

Keldeo here, a water/fighting unicorn, is actually an extra addition to the fighting trio of Cobalion, Terrakion and Verizion, based on the character d'Artagnan, youngest of the Three Musketeers. It's not too bad as a design - if you're into small, colorful horse-like creatures I guess, certainly better looking and less creepy than other fictional characters to fall under that description.

Why Keldeo necessitates an alternate form, I have no idea. It's not a bad design either, but like most legendary form changes, it feels like it only exists for the sake of padding things out. All else I can really say about Keldeo is that a unicorn at least makes a lot of sense for a legendary pokemon, and I guess it fits in well enough alongside all these dang dragons everywhere.