In Much Ado About Nothing, act 1, scene 1, Benedict talks about having a recheat winded in his forehead and a bugle in an invisable baldrick, and...

You're right on the money. Benedick is convinced that any woman he married would cheat on him. This would make him a cuckold, and cuckolds were indeed depicted as having horns. (Incidentally, there was another term for cuckolds in the Renaissance: wittol. The distinction is that cuckolds didn't know about the adultery, while wittols knew and tolerated it.)

Let's hit the language first, because Shakespeare's having a lot of fun here. A "recheat" is a hunting call, played on a bugle. It's sounded to call back the hounds tracking the prey. So not only is Benedick creating the image of physical horns on his forehead, the universal symbol of cuckoldry, but he's also implying that he would need to ask his wife to stop cheating on him. That would not be a fun conversation.

A baldrick is a belt that goes over one shoulder and holds something that needs carrying on the opposite hip. A hunter would hang his bugle on his baldrick. So suddenly Benedick has transformed the bugle from a metaphorical horn to a metaphorical penis. If his wife disappeared from home to hang out with other men, Benedick wouldn't have a place to stick his bugle, hem hem. It's interesting to note that by comparing women to baldricks, he might be suggesting they surround their lovers, wrap them up, and hang all over them. He's clearly not into the clingy types.

Note too that Benedick is using this whole extended metaphor to associate love with hunting ... which might give us a bit of insight into why his love life hasn't been very successful.

Now to "cuckold" itself. The work comes from "cuckoo," the bird that notoriously lays its eggs in other birds' nests so they will do the work of raising its chicks (the official term for this is "brood parasitism"). The connection is pretty obvious—the cuckold's wife is out getting impregnated by other men, so the cuckold will be stuck raising a child that's not biologically his.

The explanation behind the horns is a little harder to track down. A popular theory is that they represent deer horns. Male deer fight to establish dominance; the winner gets the girls. The cuckold, like the defeated deer, has no chance to mate. Another theory suggests that the horns hark back to Roman soldiers. Victorious soldiers were apparently given horns to wear as a symbol of their glory. But soldiers are away from home a long time, and, well, what's a girl to do while she's waiting? 

A pretty wild speculation comes from the fact that, back in classical antiquity, when people castrated roosters (to make them capons—better for eating), they also cut off the roosters' spurs (back claws) and grafted them to their combs (the crests on top of their head). The poor creature would look like it was walking around with a floppy set of horns. Cuckolds, like capons, are castrated in the sense that they're not able to reproduce. No baldricks available for their bugles.