bull

This is about as descriptive a word for an animal as you are ever likely to come across. What’s the main difference between a cow and a bull? It’s what sticks out, of course. However, while a cow is not defined by its udder, a bull is most definitely defined by its testicles. The word derives from an ancient root meaning “swell”, which also gives rise to ball and the Old English word for testicle, bealluc. Enough said.