Aquatic Hydra

Breath Water. The hydra can breath in water and hold its breath for 1 hour on land.

Multiple Tentacles. The hydra has five tentacles. While it has more than one  tentacles, the hydra has advantage on saving throws against being blinded, charmed, deafened, frightened, stunned, and knocked unconscious.

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

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

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

Multiattack. The hydra makes as many slam attacks as it has tentacles.

Slam. Melee Weapon Attack: +8 to hit, reach 30 ft., one target. Hit: 10 (1d10 + 5) piercing damage.

Grapple: The hydra can use two  tentacles to attack one target if both hit the target is grappled and it makes an immediate wringing attack causing 2d10+5 damage.