The Chain Chomp is an enemy and friendly character that appears in multiple games in the Legend of Zelda series. In Four Swords, they are actually an equipable item that can be used to defeat enemies.
A Link to the Past
In A Link to the Past a pair of Chain Chomps can be found within Turtle Rock, the seventh dungeon in the Dark World. These Chain Chomps will have their chain attached to a stationary block while they bounce around. After bouncing a few times, they will see Link and try to charge at him. The Chain Chomp can only extend as far as its chain allows it to.
Chain Chomps can be quite damaging, doing three hearts of damage to Link, even when he is wearing the Blue Mail. The actual chain does not damage at all and only the ball can harm Link. None of the items in Link's inventory can damage a Chain Chomp, so they cannot be defeated. Link can use the Cane of Byrna or the Magic Cape to sneak by these enemies without taking damage.
Unlike its predecessors, Link can obtain the Chain Chomp as an item received only in Vaati's Palace that can be either be an ally to him or an enemy to other Links in Four Swords. The Chain Chomp acts similar to BowWow in Link's Awakening when Link is walking it. However, like most items received in Four Swords, the Chain Chomp is not a permanent item.
Four Swords Adventures
Chain Chomp from A Link to the Past.