Missingno. theory
If you every played pokemon, then you should know a glitch known as Missingno. Most people think Missingno. is a unknown number in the pokemon game (AKA not a real pokemon script.), but it's acturly a failed clone of mew.
Now mew has been cloned once for the birth of mewtwo. But was mew ever cloned twice? Most likely. . . Yes. Missingno. Was going to be mewthree. His code was perfect in the game, intill a error happened and corrupted it's code. The creators tried everything to fix the glitch, but nothing helped. Due to there failed attempts, mewthree's sprite was changed into a glitch sprite. Even the name changed from mewthree to Missingno. They tested the game to see how glitched mewthree was. About 5 minutes afterthe first battle, the creators of the game removed Mewthree. Some how Mewthree stayed in the game. If you do the Missingno glitch then you will find The glitch pokemon.
Why do we find Missingno at Cinnabar island?
We find Missingno. there, because Mewthree was going to be the legendary pokemon at that city. He was going to be on a small island that the move surf could get you there. When they tryed to remove Missingno. they removed the island, because there will be no use for it.
Mewthree/Missingno.'s original type and moves.
Mewthree was going to be a psychic and water type. His moves where going to be watergun, fly, psybeam, and psysky. Psysky is a move only mewthree could learn. In this move, mewthree flys in the sky for 4 turns. Every turn a weak psychic attack happens to the enemy. Gamefreak tried to remove this move. That just corrupted the whole pokemon. Other then that, mewthree could learn any TM.
Mewthree/Missingno. in the show
Because mewthree couldn't be in the game, they wanted to add him in the show. The art of mewthree looked like a mew with two tails, a robotic eye, white fluffy fur, and a robotic front leg. They day they where going to add mewthree into the show, a angry mob was banging on the studio's door. Someone took a pic of there mewthree art and have posted it on the internet. Turns out, no one likes or knows mewthree, so they never added him the show. Instead, they put Ho-Oh in that episode.