Q is a glitch Pokémon in Pokemon Yellow.
Q can be obtained by doing Method #3 of the Mew Glitch with a Special stat of 192 to get a 4 4 Hy, which evolves into Q at level 6. Q will have the nickname "TM55" after evolving from from 4 4 Hy. When viewing its stats, it will have the number #121 and its types are Water/Psychic. When traded to Red and Blue, it will become another glitch Pokemon, called Charizard 'M.
Q hides all Pokemon underneath it in the party or box. Any Pokemon hidden by Q cannot be healed at a Pokemon Center, including Q itself. They can only be healed using items until Q is switched to another position that makes the Pokemon visible again.
Caution should be taken before depositing a Pokemon above Q with Pokemon below it. This can cause unstable hybrid Pokemon to be created. Q should be moved to the bottom of the party before any Pokemon are deposited. Also, Q cannot be in another box with other Pokemon, or it will make hybrids of Pokemon inside the box when they are withdrew. Players with one or multiple Q should store them all in their own box.
- http://bulbapedia.bulbagarden.net/wiki/Q - Q's page in Bulbapedia