UltraMew was a regular Mew, living in a forest in Guyana. However, one day, he was captured by a team of evil scientists. These scientists had been working on a failed Team Rocket project, to use radio waves to make a Pokémon evolve.
After capturing Mew, they subjected him to these radio waves. It appeared to have no effect at first, but slowly Mew evolved into an entirely new species, UltraMew!
UltraMew then learned UltraTeleport, and escaped from captivity. He has been fleeing ever since. The admins of PokéCommunity offered him safe haven, and he's been here ever since