That

That was an enormous B-movie monster which appeared in the Ben 10 animated series episode Creature Feature.

Description
At first appearing as a large, humanoid, reptilian monstrosity with green skin, a lashing tail and multiple eyes when overfed on light, That metamorphosed into an even larger beast with four legs, large thorn-like protrusions, a tongue studded with eyeballs (which it used to consume even more light) and three heads.

History
When Team Tennyson visited a drive-thru to watch an old monster movie called That, they were treated to a show like no other, as power-hungry magician Hex appeared and brought the titular creature from the screen into real life. Ben, assuming that as the beast fed on light it would be overloaded and destroyed by exposure to too many rays, used his Diamondhead form to enhance the amount of light given off by nearby stadium lamps. However, this plan backfired, as That simply kept on absorbing the beams.

What the Tennyson's were unaware of was that Hex had summoned That as part of a ritual which he must enact every one hundred years: the beast would absorb a certain amount of light, and in turn Hex would absorb the beast, this act renewing his magical powers and sustaining his immortality. When Ben overfed That, it made the creature too powerful for Hex to call back and absorb.

As a result, Hex was forced to make a truce with Ben (who had switched into his Wildvine configuration), and together the pair succeeded in weakening That to the extent where Hex was able to finish the ritual and banish the monster. Afterwards, although Ben's Omnitrix had timed out and left him at the mercy of the newly-repowered Hex, the sorceror was unable to finish of his foe as the code of his magic prevented him from using it against anybody to which he owed a life-debt.