Goop is a gelatinous creature of intermediate species. Goop can stretch and shape shift and reform limbs due to his gelatinous form (hence the name Goop). His slime is controlled by a small UFO like object, which is also where his voice comes from. However, if the UFO is separated from the slime from even a small amount, then the slime will collapse onto the ground, in which the UFO must collect up the slime for it to have structure again. It is possible that Goop is an organic form of Upgrade. Goop can shoot out acid as seen in "good copy, bad copy".It has been hinted that the acid is used for goop to digest his food,showing that goop might be two lifeforms, an ameoba and the UFO-shaped robot that controls the brainless ameoba.
Goop is probably based on Upgrade by his ability to morph and shape shift.