This item drops from:
- U'cha in Un'Goro Crater
- Un'Goro Thunderer in Un'Goro Crater
- Un'Goro Stomper in Un'Goro Crater
- Uhk'loc in Un'Goro Crater
- King Mukla in Stranglethorn Vale
This item is a reference to the "Donkey Kong" series, as in the original game, the title character threw empty barrels at the player.
It could also be a reference to an old toy called "A barrel of monkeys" in which there were many small plastic monkeys inside a plastic barrel. The idea being that there are monkeys everywhere because they all got out. See: Wikipedia:Barrel of Monkeys (game)