Clumsy Oaf. Any Attack or Spell that would cause Petey to be knocked Prone is made with Advantage.

Magic Resistance. Petey has advantage on saving throws against spells and other magical effects.

Multiattack. Petey Piranha makes three attacks of his choosing. 

Headbutt. Melee Attack: +6 to hit, reach 15ft., single target. Hit: 10 (2D6 + 4) Bludgeoning damage. (Petey lunges forward 10ft to deliver the Headbutt)

Bite. Melee Attack: +6 to hit, reach 5ft., single target. Hit: 17 (3D10 + 2) Piercing damage.

Spew Sludge. Petey spits out a ball of toxic sludge in a 60ft range covering a a 10ft radius of terrain in the disgusting substance. Creatures in the area of impact must make a DC 14 Dexterity saving throw or be slowed by 10ft. Creatures ending their turn while in the affected terrain take 1D4 Acid damage.

Whirlwind. Each Creature in a 30ft cone radius must make a DC 16 Strength saving throw. On a failure the target takes 2D6 bludgeoning damage and is flung 30ft away from Petey in a random direction and knocked Prone. If Creatures collide with an object they take an additional 1D6 bludgeoning for every 10ft it was thrown. If the target is thrown at another creature, that creature must make a DC 12 Dexterity saving throw or take the same damage and be knocked prone. On a successful save the target takes no damage is only pushed by 5ft.

( Can only be used while Flying )
Cannon Ball. Melee Attack: +6 to hit, reach 60ft., single target.  Petey rolls into a ball and falls slamming his entire body into the ground. On impact Creatures take 1D10 Bludgeoning damage per every 10ft of height Petey was at. Creatures in a 5ft radius of the impact but make a DC 10 Dexterity saving throw or be knocked Prone.

Steaming Angry. Once Petey drops below half his maximum Hit Points he gains 20ft movement. (Walking, Flying, and Burrowing)

My Turf. Petey upon being confronted by enemy/hostile Creatures will let out an ear piercing wail of a roar that can be heard in a 200ft radius.