Brontohydra

Multiple Heads. The brontohydra has five heads. While it has more than one head, the brontohydra has advantage on saving throws against being blinded, charmed, deafened, frightened, stunned, and knocked unconscious.

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

At the end of its turn, it grows two heads for each of its heads that died since its last turn, unless it has taken fire damage since its last turn. The brontohydra regains 10 hit points for each head regrown in this way.

Wakeful. While the brontohydra sleeps, at least one of its heads is awake.

Stomp. Melee Weapon Attack: +8 to hit, reach 20 ft., one target. Hit: 27 (5d8 + 5) bludgeoning damage, and the target must succeed on a DC 14 Strength saving throw or be knocked prone.

Tail. Melee Weapon Attack: +8 to hit, reach 20 ft., one target. Hit: 32 (6d8 + 5) bludgeoning damage.