The Powder Keg is one of the most powerful explosives in the Zelda series making it's only appearance in Majora's Mask.
Though it's not easily obtained. You still have to complete a good half of the game, and win the goron race as Goron-Link. Which isn't a walk in the park to say the least. But after a tough adventure, it can be done.
The sad thing is that it's only real good use is to destroy a rock blocking the path to the Romani Ranch. It's swell to get Epona back and do it's side-quests, but it's a bummer that it can't be used that well in battle.