The Dunkin' Dumpster is a Playset that was released alongside Series 2. It comes with 2 Exclusive Trashies, Slime Bucket and Mouldy Mushroom
and has holes to place your Trashies in. The concept of this set is to try to hit the target and dunk the Trashies. The back of the packaging shows Slime Bucket standing in a bucket, which could be why Slime Bucket was chosen to be the exclusive Trashie and Mouldy Mushroom is seen hitting the target, hence his big head.
Trivia
Even though it's a Series 2 release, it includes Series 1 trash cans for the trashies.