In honor of my childhood and to coincide with the release of the new TMNT movie (which I have hopes for, but is probably going to be bad), I present to you the kappa for 5E.
Kappa are strange humanoid turtles from the far east. Their most bizarre feature is the head bowl at the top of their skulls. The basin of this is filled with water from the kappa's home water source. Kappas are often either playful and puckish or cantankerous and ornery. They enjoy playing pranks and wrestling. Kappa fear losing water from their head bowls. If they do so they are unable to move and rendered sluggish. They will beg for any around them to refill the bowl. Anyone who does so gains an ally.
Medium Humanoid, Chaotic Neutral
Armor Class 15
Hit Points 30 (4d8+8)
Speed 30, Swim 30
STR DEX CON INT WIS CHA
12 (+1) 17 (+3) 13 (+1) 8 (+-1) 15 (+2) 10 (+0)
Skills Perception +4Senses dark vision 60 ft., passive Perception 12
Challenge 1 (200 XP)
Head Bowl. The water in a kappa's head bowl cannot be emptied by anyone other than the kappa (wither it does so willingly or is forced to do so while pinned in a grapple). If the water is emptied the kappa gains the Restrained condition until the head bowl is refilled. The water doesn't have to be from the kappa's water source, but the kappa will refill it from that place as soon as possible.
Multiattack. The kappa makes two melee attacks.
Claw. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 4 (1D6+2) bludgeoning damage.
Claw Rake. If the kappa hits the same target with both claw attacks it deals it deals an additional 4 (1d6+2 damage) to the target.