Occasionally two monsters will be incompatible and just won't blend. When this happens, a Sloodge is created.
Technially I don't want to paint myself into a corner and start ruling out combinations that I might have ideas for in the future, but the vast majority of tertiary monsters don't blend and I can say with certainty that two Rarebits won't work either!
When blending primaries and secondaries Sloodges are actually quite rare.
But they do count as a primary monster so they can still be blended into other things.