Interesting topic because most of what we think the Bible says about the Devil isn't in the Bible at all. It's all later church interpretation and interpolation. For example, in the Old Testament book of Genesis nowhere does it say that it was the Devil that tempted Eve to eat the forbidden fruit (and nowhere does it imply that this fruit was an apple). The culprit is simply referred to as"the serpent" throughout. It's only theologians of a much later age who decided that "serpent" must equal "devil."
We don't really get much (if any) mention of something that could legitimately be called "the Devil" until the New Testament. It does say that when Jesus spent 40 days meditating in the desert, the Devil came to tempt him. But this is a faceless sort of demon, lacking any personality or discernible evil traits.
The Devil as we know of him is really the invention of the Medieval priesthood. I don't know just where the notion of the tails and horn came in. Maybe Wikipedia has something on that; I'll have to look it up. There is a further problem about identifying the Devil in old Christian writings because they were mostly written in Latin. Now the Latin language largely dispenses with such niceties as definite and indefinite articles, something we're used to seeing in English. So later translators, running across a word like diabolus
in a Latin text had no way of telling whether the original author meant to say "the Devil" or "a devil," referring to an insignificant evil spirit, an imp.