i'm in your conscience
It's not a real Pokemon. It's just a glitch that appears for...well, it has something to do with the Pokedex number code on the wild encounter Pokemon going past 151. This will explain it better than I can: http://bulbapedia.bulbagarden.net/wiki/Missingno
Even better: http://www.youtube.com/watch?v=HQeedFtYckw
Just know it's not a real Pokemon. More like a glitch Pokemon, so it doesn't get a number (fitting for the name, eh?) or a dex entry.