Bulls should spawn and follow cows. If a cow is hit in the presence of a bull, the bull will charge after the attacker.
When a bull hits its target, it launches them into the air like the iron golem.
To tame a bull you need to feed it hay bales or the player can repeatedly mount (like the horse but the player risks being launched high into the air). After it is tamed, you can saddle and ride it but instead of jumping you can make it charge forward. If it hits a mob while charging forward, the mob will be launched into the air.
Bulls drop 3-6 leather and rarely one hide.
Hide can be used to craft armor as tough as iron and which can be dyed like leather. While wearing the armor and sprinting the player can use the bull's charge ability.