Missingno.
Missingno, also known as the Cinnabar Anomaly, was a Glitch Pokemon, and the first major villain of WAAPT.
Origin
Missingno was originally intended to be a second attempt at Mewtwo, but it went horribly wrong and it was set free into the waters surrounding Cinnabar Island, remaining little more than an urban legend for years.
Sinnoh Arc
Missingno for unknown regions made its way up from Kanto to Sinnoh, where the proto J-Team encountered it shortly after defeating David and Leena, and after it turned Canalave City into a batch of Bad Eggs. After a portal was opened above Mt. Coronet, they did battle for the first time, when Missingno was temporarily defeated by prayer. Afterwards, it possessed a number of legendaries, including Palkia, and transported all of the proto J-Team to PMD-A
PMD Arc
Missingno met its end during the PMD Arc, after using the legendaries it had possessed in an attempt to kill the trainers, who were transfigured for the majority of the arcc. Though it managed to nearly defeat the proto J-Team using their insecurities, Anom proved to be above the influence, and everyone combined their attacks to finish off the glitch. Shortly before it died, it revealed its wish to be a regular Pokemon, which moved the trainers so much that they buried its broken body before having Palkia transform them back.
Powers and Abilities
A biomechanical monstrosity fused with a supercomputer, Missingno was capable of warping reality simply by existing, and was powerful enough to even bring Pokemon as powerful as the creation trio under its thrall. It also had the ability to summon photosensitive shadow creatures to serve as minions, and was capable of reading minds in order to invoke mind rape.