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The church (or I guess individual evangelists) generally picked up existing customs and references wherever they went.
So if you have a goat-footed good-time god that encourages un-Christian behaviour, they get identified with the devil. That mid-winter piss-up you were having anyway is now apparently Jesus's birthday, etc.
Anyway I'm with William Blake up there: Lucifer is definitely meant to be a handsome devil. Beelzebub and the others might have arse faces and all sorts, though.
Read something once that the church depicted him as goat like so all the peasants stopped worshiping Pan...the old pagan god or something who had horns. The church thought that then the people would believe in Christianity as they would think Pan was evil and not good. But what do I know...I like the thought of the devil being goat like though, as then his head fits into a pentagram nicely.