Satan was goated as a callback to Pan, one of the chthonic Hellenic gods (not to be confused with the Cthonian Hellenic gods. Ia! 🐙🌊🌠).
Late stage Jesus was Apollonian rather than Dionysian so Satan, now a proper antagonist, inherited all the chthonic stuff.
It's also why (Pauline Paulonian as in the Apostle Paul -- I was thinking of Apollonian as above. Words.) Jesus is about asceticism where Satan is about indulgence.