Sludgepuppies

From Wikitrix

Sludgepuppies are aliens that have fought the Plumbers on a regular basis. The Sludgepuppies appear in the episode "My Big Fat Alien Wedding". They are giant, mud-based creatures that can shapeshift into humanoid forms. Their natural state is a legless, vaguely humanoid blob. Females of the species have antennae in this form. They can stretch their limbs and create makeshift weapons from their composite mud. Being made of a mud-like substance, however, makes them vulnerable to water and fire.