The Slime Monsters are characters who repeatedly show up in Stale Pancakes and try to eat all the planets the main characters go to. Notable members include Dave the Slime and Slime Monster King.
In Stale Pancakes Oldies, the Slime Monsters invade Rainbow Valley and are fixed into a war with the main cast. Toilet Chompers are invented to quell their forces, and if you feed them hot wings they erupt into flame. Some slimes are seen to have wings, such as when they melt Ice World.
It's kind of not exactly clear what the slimes are doing. What is clear is that their king wants to "engulf", i.e. eat, all of the worlds. So what are the Slime Monsters there for, then? They don't eat anything. Are they there as heralds that the king is coming? Are they there to raise mischeif of their own, as maybe children or spawn of the king?
In Stale Pancakes 7, the slimes serve very much the same role as in Oldies. Their takeover of Rainbow Valley is very much a natural threat, and their king incidentally turns into a world-eating giant.
It is seen in 7 that there are different types of slimes. On Meadow World, there are larger, more lumbering and dumb Slime Monsters who the characters slink by with ease. These probably were set up to be a bigger threat that never came, because the Slime Uprising saga never got finished.
Slime Monsters have attacked worlds and tried to engulf all worlds numerous times. The twelfe time they ate PC they proceeded to eat all worlds until they went to the moon and Willy fell in a slime pool that just turned out to be a portal to a slime world, where everyone lives today. (see "2-28-12" for [lacuna])