The hider

Basically a 1.5 tall drowned creeper, it swims like a squid but less buggy. It "explodes" in a weird way, so it damages you and removes some water in the damage radius but on land it adds water with an inverted algorithm. When on land it does that hopping thing like a dolphin, but it will only even get close to air if a player moves out of water while it follows a user.