Arthur

Natural Defenses. Arthur AC increases by 1.

Powerful Attack. Arthur basic attack increases it's damage dice by one size.

Reckless. At the start of its turn, Arthur gain advantage on all melee attack rolls it makes during that turn, but attack rolls against it have advantage until the start of its next turn.

False appearance. While motionless, Arthur is indistinguishable from a rock.

Headbutt. Melee Weapon Attack: +4 to hit, reach 5 ft., 1 target. Hit: 1d8 + 2 bludgeoning damage. 

Protective. When a creature Arthur can see attacks a target other than you that is 5feet of it it can use its reaction to impose disadvantage on the attack role.