Smaller elemental creatures like fire elementals and fire demons come together to form a greater fire demon or something like that. Works equally well for water, air, and earth creatures.
As mentioned, slimes and amoebas could join together to form a titantic, corrosive, devouring blob.
In graveyards heavily tainted by chaotic magic, skeletons could disassemble themselves to form a skeleton dragon or other large monster made entirely of bones.
In an area like Darkforge, a corrupted metal working place, animated armor and animator weapons could merge together to create titans composed of steel weaponry. Ouch.
Such inverse breeders could make cool bosses for quest completion.