bestia

Bestia, bestiae is first declension feminine, and it is the grammatically easy member of an awkward set. Animal is any living creature including humans; fera is a wild animal as opposed to a tame one; pecus is livestock; bestia is a beast, usually large, usually dangerous, and specifically the sort put into an arena.

That last sense gives the word its Roman colour. Damnatio ad bestias was execution by wild animals, and bestiarius was the man sent in to fight them, which is the background to every martyr narrative in this corpus that has a saint facing the beasts. Christian writers then took bestia as the standard rendering of the Beast of Revelation. English bestial and beast come from it; note the plural bestiae and dative bestiis are one letter apart from the singular.