Cerbearus, Owl

Keen Sight and Smell. The cerbearus has advantage on Wisdom (Perception) checks that rely on sight or smell.

Reactive Heads. For each head the cerbearus has beyond one, it gets an extra reaction that can be used only for opportunity attacks.

Multiple Heads. The cerbearus has three heads. While it has more than one head, the cerbearus has advantage on Wisdom (Perception) checks and on saving throws against being blinded, charmed, deafened, frightened, stunned, or knocked unconscious.

Whenever the cerbearus takes 25 or more damage in a single turn, one of its heads dies. If all its heads dies, the cerbearus dies.

At the end of its turn, it grows all of its heads back, unless it has taken fire or acid damage since its last turn. The cerbearus regains 10 hit points for each head regrown in this way.

Wakeful. When one of the cerbearus' heads is asleep, the other two are awake. 

Multiattack. The cerbearus makes as many beak attacks as it has heads, and one claw attack.

Beak. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 10 (1d10 + 5) piercing damage.

Claws. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 14 (2d8 + 5) slashing damage.