Rat Hole

From Zelda Wiki.org

Rat Holes are small holes in the wall that serve as homes to rats in both The Wind Waker and Phantom Hourglass.

The Wind Waker

A set number of rats will appear out of these holes, but if you toss some All-Purpose Bait in front of it, a rat will come out and offer to sell you something. It sells things such as potions and more All-Purpose Bait. This raises the question of why they need Link's All-Purpose Bait when they already have more then enough to sell...

Phantom Hourglass

Only four of these holes appear in game, and they are in pairs. A rat holding a small key will appear from one hole and run to the other. In order to get the key from the first rat (Located in the Mountain Pass on Mercay Island), Link had to push a block in front of one hole, then let the rat run to it and corner itself. In order to get the key from the second rat (Located in the Temple of Fire on the Isle of Ember), Link needed to kill it with his Boomerang while it was running from one hole to the other.

Similar holes also appear, however, and serve as doorways only accessible by Bombchu.