Bulls are rare in Red Dead Redemption, appearing individually only in a couple of areas. Bulls can be hunted, but killing a penned bull counts as a crime. They are distinguishable from the other cattle by their large size, and they will charge the player if approached, knocking them down.
There is a bull located in MacFarlane's Ranch, in the cattle pen. Be warned, if you kill the bull in the cattle pen, you will become wanted! Another can be found in and around the Twin Rocksgang hideout.
There is a glitch that can occur in singleplayer that has a horse appear as a bull. This allows the player have a bull mount in singleplayer. The "bull" however, makes the same sounds as a horse and still retains the "ride horse" prompt. It also gives the ability to jump.
Sometimes, if the bull charges you while you're outside the pen, it may go through the fence. This way, you can kill it outside of the pen without getting wanted.
Bulls are one of the only animals in the game to have correct anatomy.
Sometimes, bulls will attack NPCs located in MacFarlane's Ranch. Ranch hands that enter the pen may be run over by the bull. Also, if the bull becomes agitated and escapes from the pen, he will charge at anyone, whether it's the player or an NPC, that comes near him.