Horns show up consistently in the play as a symbol of marriage (and the corollary notion of a husband being whipped in marriage). Remember, cuckolds were men who were married to unfaithful wives, and were generally long-suffering. Horns are an interesting symbol, because while they’re used in this context to denote a whipped guy, they’re also a thing of wild beasts. Don Pedro jokes about the "savage bull" with horns at the beginning of the play when Benedick refuses to marry, and again when Benedick has chosen to marry after all. The inevitable horns that come with marriage (remember Beatrice jokes that if God sent her a man, He’d have to send horns too) represents marriage as a process by which wild animals are tamed.
Horns are joked about throughout the play, but over the course of the action they become less fearsome as the characters come around to viewing horns as an ornament of love. At the end of the play, Claudio promises that Benedick will have horns, but they’ll be gold-tipped, like Jove’s horns were during his lusty conquest of Europa. Horns are thus a thing of wild animals, but as the characters learn to accept and be excited about marriage, they come to symbolize the bawdy pleasantries of marriage.