Bull

Rage. Once per short rest, the bull can enter a rage.  During the rage, the bull gets a +6 Armor Class bonus and a +2 attack roll advantage.  When raging, the bull cannot be Frightened or Charmed.  The effects of the rage last for 3 rounds.  To enter a rage, the bull must make an Intimidation Check.  The Intimidation Check value must be higher than 15. 

Pain-Fueled Anger.  When the bull's health reaches levels below 15, he gets a +3 advantage on Intimidation Checks.  This makes it easier for the bull to enter a rage.  

Charge.  Once per round, if the bull is at least 20 ft. away from the opponent, the bull can charge, moving up to 35 ft. per movement action.  Charging gives the bull a +4 advantage on the first attack used after using the Charge action.     

Headbutt. Melee Weapon Attack: +6 to hit, reach 5 ft., 1 target. Hit: 1 (2d8 + 4) piercing damage.  If Headbutt is used after charging, the opponent must make a successful Strength Saving Throw or be knocked Prone and pushed back 5 ft.  The Headbutt attack can be used as a bonus action after using the Charge action.            

Bite. Melee Weapon Attack: +2 to hit, reach 3 ft., 1 target. Hit: 1 (1d4 + 1) piercing damage.

Rage. Once per short rest, the bull can enter a rage.  During the rage, the bull gets a +6 Armor Class bonus and a +2 attack roll advantage.  When raging, the bull cannot be Frightened or Charmed.  The effects of the rage last for 3 rounds.  To enter a rage, the bull must make an Intimidation Check.  The Intimidation Check value must be higher than 15.