Donny is a green grass ogre from the Adventure Time episode of the same name, "Donny".
He acts mean to the building people who share living space with him. Finn and Jake bring Donny to their house and turn him into a nice person. However, the Why-Wolves tell Finn that Donny's attitude keeps them from attacking the building people, and without him they will wipe out the inhabitants.
Finn and Jake return with Donny, who is now nice and interested with singing. They make fun of his singing until the humiliation reverts him back into a jerk. The wolves flee and Donny goes back to bullying the building people, proud of it.
Got to get pants. Go get your pants Button Fly!