So, Chorus is an Extraterrestreon, from exoplanet 42 Draconis B. The thing about 42 Draconis B is that it's basically a giant evolution stone, so basically the moment the Eevee there hatch, they evolve. Instead of fur, they're covered in highly flexible and adaptable tentacles, kind of like rubber, and are invertebrates, so they don't really have, well, bones. This means they can fit into disguises quite easily, for instance, Chorus's classic Rockruff state. The tentacles have practically no pain receptors in certain points, so they can be cut off easily, which is why Chorus is rather less tentacled than others.
She used to be an upbeat youth, though she was always cast aside due to their albinism(which reacts quite differently to an Extraterrestreon's body than other creatures) and began to slowly but surely lose it. Eventually, she isolated herself entirely until her mind was at last gone. In her mania, she'd become one who cared no more for identity but for thrill, a court jester in her own little world.
Caption this Meme