Read my explanation on the multiplayer leveling before reading this.
1. Getting to level 50 first time - Bonzo (a regular bull)
2. Getting to level 50 second time - Buffalo (standard dark brown bison)
3. Getting to level 50 third time - Albino Buffalo (whitish bison)
4. Getting to level 50 fourth time - Super Bull (a special bull I guess, I don't know much details about it)
5. Getting to level 50 fifth time - Zebra Donkey (a donkey with zebra stripes, is the fastest thing ever)