(gods, why is this so much better looking than the 3D model? It's the non-diseased-looking forehead, isn't it?)
Mega being Revolved: Mewtwo-Mega-Y
New Pokémon Name: Corruptoo ("corrupt(ion)" + "too" as a pun of typoing "two" due to both being homophones with each other)
Stats: 106 HP / 90 Atk / 090 Def / 160 SpA / 125 SpD / 159 Spe (-00 / -20 / +00 / +06 / +35 / +29)
Typing:
/
Abilities: Insomnia /
Corrosion (new; hidden ability)
Movepool Additions: Sludge Bomb, Sludge Wave, Toxic Spikes
Niche: A stronger, faster, terrifyingly special attack that I wouldn't be surprised if it is rather broken given it still currently has access to Calm Mind in addition to Psystrike and other things unfortunately. I gave it a Poison-typing almost solely because of how much I dislike its visual design; it's easily the Mega I dislike the most on that level. Poison-typing does at least fit thematically as the other type Psychic preys on besides Fighting though, and when it comes practical aspect its additional Poison-typing is at least decent, preventing it from being Poison'd, letting it reasonably have more support in Toxic Spikes, and giving it a quad-resistance to Fighting at least even if the all of the premiere Fighting types in Uber have other ways to physically murder it; it's also a great Geomancy Xerneas check I imagine if that mon is even still a thing.
As for Corrosion, if nothing else it allows this to at least not be basically walled by Steels if it's just relying on its STABs since no-miss Toxic basically screws over all of them. Finally, while Poison is still admittedly such an underwhelming attacking type that it was also chosen for that reason since it's probably the one that Corruptoo can do the least additional damage with as a whole, this mon blows up other Poison mons via its Psychic typing like Eternatus. (The weird Speed is so that Ninjask can pretend it still has worth as Ninjask continues to ignore the existence of Deoxys-Attack and now Regieleki; in all seriousness, I just wanted to be faster than Ghost Calyrex and Pheromosa while adding to its other stats the least.)