The following information is from the Adventure Time comic book series. It may not be part of the same canon or continuity as the animated series.

Lumpy Space Predators are animals who live in Lumpy Space. One of them, a Lumpy Space Rat, attacked a group of Breakfast Organisms when Lumpy Space Princess brought them to Lumpy Space while their home was growing back. There are presumably more varieties besides the Lumpy Space Rat.