Are ultra beasts legendaries or not?
I’m not sure it matters? Being considered “legendary” doesn’t really imply any particular status, and they don’t follow a consistent set of rules. But… sure. There are certainly legends about them in Alola, and they seem to have been unknown outside of those legends until fairly recently, so in that sense they are literally “legendary.” They aren’t one of a kind, but then again, neither are Heatran, Lati@s, Manaphy, etc. In game terms they certainly have stats commensurate with being called “legendary.” I think probably a much more interesting question is whether they are Pokémon or not, and as far as Game Freak is concerned the answer seems to be “yes,” but I suspect that this implies some less-than-straightforward further questions about what a Pokémon is.