Sentinels are giant robots generally programmed to hunt, capture, and sometimes kill mutants in the Marvel Universe.
Sentinels were developed by Dr. Bolivar Trask to combat the so-called "Mutant Menace." When they were first unveiled, they promptly decided the best way to protect humans from mutants was to take control of the world. Dr. Trask eventually helped the X-Men shut them down, but he and other overzealous mutant-haters continue to construct them even now.
Sentinels appear in many different forms, and the COTA-94 model is very distinct. Their standard form is a large, humanoid robot, about 40 feet tall. They are colored pink and purple as standard. In Marvel vs. Capcom 2, a mango-colored Sentinel color scheme became very popular color scheme after a YouTube video referenced it. The alternate color reappears in Marvel vs. Capcom 3.
In the Vs. series, Sentinel is considered to be one of the best characters and has the most health of any character in MvC2 or MvC3. He his, however, very slow-moving. His character model is also the largest of the entire roster in both games (excluding boss characters).
He uses sentinel drones, a rocket punch, and lasers from his mouth to keep opponents at bay. Many of his normal attacks have armor, which allows him to ignore hitstun from many attacks and continue huge damage combos of his own. In MvC2 especially he was known for his high mobility flight mode and long range attacks while flying around the stage or running away.
The COTA-94 model designation was not officially assigned to the Sentinel in Capcom fighting games until Marvel vs. Capcom 3. It references the model's debut appearance, which was X-Men: Children of the Atom in 1994.