With Toei there's 2 layers:
Oda -> Toei scriptwriters -> Anime translators
With VIZ it's:
Oda -> Stephen
No one can take Animal Kingdom Pirates serious compared to Beasts Pirates.
Technically not wrong -> It's 獣. Although Beast would work as well.
I think he translated it as "Animal Kingdom", because it's technically Hyaku-ju, which is also Kaido's title. "100 animal/beasts" is an expression meaning all sorts/types of animals. So Kaido would be the ruler of all sorts of monsters.
But yeah Animal Kingdom sounds like something out of a Disney movie. Doesn't sound threatening. Although it's more accurate.