Bari are enemies found in The Legend of Zelda: A Link to the Past, The Legend of Zelda: Ocarina of Time, The Legend of Zelda: Oracle of Ages and The Legend of Zelda: Twilight Princess. They resemble real-world jellyfish, though are much more colossal in size. Since they are (presumably) a type of jellyfish, they'll shock any of their opponents (including Link) with a powerful jolt of electricity.
Bari were a common enemy in the dark world of Hyrule in the SNES game The Legend of Zelda: A Link to the Past. The Bari found in Ocarina of Time are creatures that can only survive inside of an already living being/host, and thus make Jabu Jabu their home (they're also found here in Oracle of Ages). In the game, the only way to kill one is to toss your boomerang at it. In Twilight Princess, you have to shoot your claw shot, which will grasp onto the Bari's brain and thrust it out of its body.