The Sentinels are a variety of mutant-hunting robots, usually portraying as antagonists to both "good" users and allied Terminators. They are far different from demons and Element Humanoids because they were originally designed to capture or kill mutants. This is only the case for the Marvel Universe. In the GoAnimate Universe, however, they only hunt people who call themselves "good users" instead of mutants.
Since they're far different from both demons and Element Humanoids, they're considered a separate type of enemy (Sentinels), which refers to a list of bad users whose true forms are revealed that bad users are actually Sentinels. In the sequel, Erika Adventures 2: Operation: Black Mesa, the Sentinels are chief antagonists midway through the film, right after the Strogg and Corrupt Terminators, and before Nihilanth himself. Their leader is Master Mold.
The Sentinels were first created by Dr. Bolivar Trask, who intended to use them to save humanity from what he saw as a threat to the species' existence in the form of mutants.In a television debate between Trask and Professor Charles Xavier, Trask revealed and then activated the Sentinels, who promptly decided that the best way to protect humanity was to rule over it themselves. The Sentinels kidnapped Xavier and brought him and Trask to the primary Sentinel, Master Mold, only for Xavier's students, the X-Men, to find them. When Trask realized the error in his ways and that not all mutants were a threat to the world at large, he aided the X-Men by sabotaging the machinery in the Sentinel base, destroying Master Mold and the Sentinels in the explosion, but he died in the process. However, numerous Sentinels and several Master Molds were built after the destruction of the original models.
In the GoAnimate Universe, however, they have the similar origin to that in the Marvel Universe, only their purpose is different; to dispose good users from YouTube so that way bad users can rule YouTube as superiors. Coulden discovers an error in the Sentinels that not only good users were a threat in YouTube at large, but also bad users, demons and Element Humanoids were also considered threats to the Sentinels, thus, becoming the new enemies to the good users. Their leader is usually Master Mold, just like in the Marvel Universe.
So far, the only GoAnimate variations of these antagonistic enemies were made by Coulden Pettit and Tigrus879.