fera

Fera, ferae, f., first declension, but it did not start life as a noun. It is the feminine of the adjective ferus, "wild", with bestia understood: a wild one. That history matters, because the neuter plural fera can still be adjectival and only the sentence tells you which you have.

Latin has several words here and they are not interchangeable. Bestia is the beast of the arena, animal any living creature including a man, pecus the domestic herd, and fera the untamed animal in the wild - game, predator, the thing outside the fence. English takes ferocious, feral and fierce from the same root, the last through Old French fier, where the sense had softened all the way to "proud".