Bari

Bari are enemies found in The Legend of Zelda: Ocarina of Time and The Legend of Zelda: Twilight Princess. They resemble real-world jellyfish, though are much more colossal in size that are found in Jabu Jabu's belly. Since they are (presumably) a type of jellyfish, they'll shock any of their opponents (including Link) with a powerful jolt of electricity as a form of offense.

The Bari found in Ocarina of Time are creatures that can only survive inside of an already living being/host. In the game, the only way to kill one is to toss your bommerang at it. In Twilgiht Princess, you have to shoot your claw shot, which will grasp onto the Bari's brain and thrust it out of its body.