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.
A Lumpy Space Rat is a Lumpy Space Predator. One tries to eat a Sausage Microbe in "Bacon Fields," a back-up story for Issue 3 of the Adventure Time comics. Lumpy Space Princess quickly saved the Sausage Microbe and the rest of the Breakfast Organisms by shooing the Lumpy Space Rat.