Ultra Beasts were based on rejected Pokémon designs
The Ultra Beasts in Pokémon Ultra Sun and Ultra Moon have always looked like they belonged to a different franchise. In the games' fiction they come from another dimension, and in real life illustrator Yusuke Omura — who worked at Game Freak until 2016 — revealed on X that he based them on designs Ken Sugimori rejected during his time at the studio.
Omura said he 'designed the Ultra Beasts by recalling some of the designs I made which were rejected by Mr. Sugimori, and I decided to deliberately use the very reasons for why they were rejected and base the artistic direction of the designs on them. So in some way, they're like a manifestation of my own resentment — pretty scary.
Of course, this doesn't mean that I hate Mr. Sugimori or anything like that.' He also replied to a fan theory that the Ultra Beasts were meant to look distinctly un-Pokémon-like, saying that was the intention and he's glad it came through.