Polymorphs are gelatinous, mucus-like, light-green, 200lb, extra-terrestrial blobs. Ben's Polymorph form, Goop, uses an anti-gravity, which is a ufo-like projector to keep itself together on planets with gravity differing from that of Viscosia, the homeworld of the Polymorph species. Polymorphs do not naturally have anti-gravity projectors, they are only for when Polymprphs travel to planets with gravity differing from Viscosia. The slime is normally in a humanoid shape with a pointed head, narrow eyes, and "spikes" on their shoulders. They can, however, revert to a puddle, a shapeless mass, or mimic the shape of something.
Polymorph (a word meaning "many shapes") is a species from the molten planet of Viscosia.
Polymorphs have strong digestive acids, which can be used as an offensive weapon, either by smothering something or releasing a sort of corrosive gas. It has been confirmed by Dwayne McDuffie that Polymorphs come in all different colors. See here. Polymorph's used to live on a different planet but it got destroyed, so they went to live on Viscosia.