In Ancient Greek, the Hebrew word שָׂטָן (satan, "adversary") is sometimes translated as διάβολος (diábolos, "betrayer") and sometimes adapted as Σατανᾶς (Satanâs).
In Latin, it seems like there would be three translation options: translate the word into Latin, transcribe the Greek, or transcribe the Hebrew. Jerome seems to have chosen the second and third exclusively, which is why we have Latin diabolus and Satanas.
But if he'd chosen the first, what would be an appropriate word? Something like inimicus "enemy" feels too tame for a fallen angel.
(This question arose while answering another question about Satanas.)
P.S. Is the #english-to-latin tag appropriate when it's really more Hebrew-to-Latin and/or Greek-to-Latin?