Cerberus

Keen Hearing and Smell. The Cerberus has an advantage on Wisdom (Perception) checks that rely on hearing or smell.

Multiple Heads. The Cerberus has three heads. While it has more than one head, Cerberus has advantage on saving throws against being blinded, charmed, deafened, frightened, stunned, and knocked unconscious.
Whenever the cerberus takes 65 or more damage in a single turn, one of its heads dies.

Sure-footed. Cerberus has advantage on Strength and Dexterity saving throws made against effects that would knock it prone.

Domesticated. Cerberus can be tamed with scratches and toys.

Multiattack. Cerberus makes a number of bite and fire breath attacks equal to the number of heads it has.

Bite. Melee Weapon Attack: +11 to hit, reach 5 ft., one target. Hit: 11 (1d8 + 8) piercing damage plus 5 (2d6) fire damage on an even roll.

Fire Breath (Recharge 5-6). The Cerberus exhales fire in a 60-foot cone. Each creature in that area must make a DC 16 Dexterity saving throw, taking 24 (7d6) fire damage on a failed save, or half as much damage on a successful one.

Frightful Presence. Each creature of Cerberus' choice that is within 120 feet of Cerberus and aware of it must succeed on a DC 16 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to Cerberus' Frightful Presence for the next 24 hours.

Bark. Pushes back all enemies 5 feet.

When Cerberus loses a head, it can do one of the actions

Whimper (takes all actions). Cerberus backs away 60 feet, and 3 harpies descend from the ceiling and join the battle.