There are three Chain Chomps owned by Lady MeowMeow. The most notable is BowWow, who is kidnapped by Moblins. After you rescue him, you get to walk him. He will eat any enemy he gets close to, allowing you to enter Bottle Grotto. A photograph can also be taken of you and BowWow in Link's Awakening DX. The other two are tiny Chain Chomps, one of which simply barks, while the other is part of the trading side quest.
A Chain Chomp can be found exclusively in Vaati's Palace, where it is a normal item. If you use it, you get to walk it, and it will eat enemies, and other colours of Links.