The Extremeasaurs are a breed of imaginary friends created by jerky teenage boys. These powerful creatures make their appearances in "House of Bloo's," and "Eddie Monster." One of them served as the tertiary antagonist of the pilot episode.
Most of the Extremeasaurs have the grotesque appearance of monsters, or in rare cases, very innocent and harmless looking, like the Extremeasaur Champion seen in "Eddie Monster." They can be exceedingly taller than Wilt in most cases, or are no more bigger than Bloo otherwise. Size does not deter the fact that these monsters are extremely dangerous; even the smallest of ones can be deadly, which is what explains the reason that they are kept within confinement.
They have such great power, fearsome looks, and almost no intelligence whatsoever because they are fighting machines. Despite that, there was one Extremeasaur which wasn't even strong enough to take a punch to the face, meaning Extremeasaurs have certain weaknesses, which Eduardo kept accidentally exploiting. Some weaknesses are that they are made of metal, and when Eduardo weeped, the monster turned to rust.
In a debut appearance, and also the first episode, "House of Bloo's," there was one powerful Extremeasaur that was locked away, and its strength required at least four people to outwit and defeat it but was released by two people with the same intentions: Duchess and Terrence, and that intent was to destroy Bloo. Duchess wanted revenge on Bloo because he cost her a chance to be adopted by a rich family, and Terrence wanted to make his brother, Mac, miserable and have him never see Bloo again. The monster had a giant spherical head, with two red eyes, a red mouth, black, razor-sharp teeth, tentacles with spikes that were also spherical (and if the tentacles were pulled apart, they would become bombs that could move to home in on a target) and despite its enormous size, the monster could move about the same speed as Bloo running. Duchess freed the beast when everyone had moved away from the Extremeasaur's cage, and she was able to free it because the key had been left on the cage's keyhole. At the same time, Terrence had gone to adopt Bloo, after trapping Mac in the closet (eventually, Mac escaped), while Duchess did the dirty work. Then, Terrence had taken Bloo, sometime after Bloo's "adoption," to Duchess in a scrap yard.
There, Duchess explained everything about how Bloo ruined her chances of being able to leave Foster's, and although Bloo apologized about it, Duchess summoned the Extremeasaur anyway, much to Bloo's horror and Terrence's amusement. In only a few moments, the beast had caught up to Bloo. When all seemed lost for the blue friend, he bid goodbye to Mac, to which Mac responded "Shut up!," he and the other three friends had come to rescue on flying unicorns and on the next screen, the Extremeasaur had narrowly missed Bloo. After a terribly awkward sentence by Mac, the Extremeasaur had resumed its pursuit of terror, now aiming for either Eduardo, Wilt, or Coco as well as Bloo.
When Wilt, Coco, or Eduardo couldn't do anything, Mac saw the tentacles and had an idea on how to subdue the beast. First, Mac ran under the monster and purposefully let himself get caught and let the monster swing him around like a ragdoll, and this was to draw Eduardo's attention and to have him yank off the tentacles. That plan's victory was short-lived; the broken pieces turned to mobile explosives and blasted Mac onto a pile of eggs, Eduardo onto a pile of tires. Then, the second phase was to have Wilt toss the bombs into eggs to muffle the explosion (Mac got that idea after seeing Wilt stack tires flawlessly for protection and seeing Coco laying eggs), meanwhile having Eduardo yank the tentacles apart until it had no more. However, that didn't stop the monster; it chased after Bloo, Pac-man style.
Mac finally had the beast outwitted when he shot a spit-ball at Terrence so that he could pursue him and Bloo. Mac and Bloo then split up and left Terrence to be pursued. Before Duchess could stop the monster, the gang threw vases at hers and Terrence's heads to knock them unconscious. Despite the fact the Extremeasaur did successfully eat them, it spat the duo right out. Terrence was left to deal with the unicorns, who held a grudge on him for calling them "girly," and Duchess was not forced to leave and was to remain at Foster's because being there was something she hated so much.
- The Extremeasaur champion from Eddie Monster seems similar to the world-famous Pokémon Pikachu, both of them being rodent-like, yellow, creatures with long black-tipped ears and electrical powers.