A wether is a ram or billy goat that was castrated at a young age. Wethers can be housed with the female sheep or goats, but intact bucks and rams usually are kept separately.
Wether comes from an Old English word for – surprise!— a castrated ram. This word has been around as long as people have been talking about sheep, which is a long time. A bellwether is literally the lead wether that wears a bell, but now it’s come to refer to any leader. The word wether is rarely used unless you’re a shepherd or a farmer, for the rest of us it’s often just the wrong homophone, whether we care to admit it or not.