I don't think Moe is "Legendary". He certainly isn't dangerous. He seems to be a typical Evolved Centaur, but he just has a name because he's kind of a mascot to Tabitha. Like if you worked on a farm you might have two dozen cows, most of them nameless but also a favorite called Bessie.
I'd love for there to be some huge quest delving deep into the rich and compelling backstory of Moe ... but yeah.
IIRC, Cook-cook the Fiend has a favorite Brahmin named Betsy or something.
As for Moe, I don't think he's anything other than Best Friend Tabitha's favorite Centaur.
And yes, there are Legendary creatures for all hostile species, I think. Cazador, Deathclaw, Giant Rat, and Gecko. Nightstalker and Centaur maybe. May not be Legendary Bloatflies though. Brahmin and Bighorners probably not.