Mine is Koffing, the Poison Gas Pokemon. It stores several types of toxic gas inside it's body, and is therefore prone to explosion without warning. It's gases are lighter than air, allowing it to float.
When first captured it knows the attacks Tackle and Smog, but as it develops and gains levels it learns (in this order) Sludge, Smokescreen, Self Destruct, Haze, and Explosion. This is believed by Koffing purists to be the truth, as this is how it is in the original line of Gameboy games, Red/Blue/Yellow.
In the sequel line however, Gold/Silver/Crystal, Koffing knows a third attack when you first capture him, Poison Gas. Also available later on is the attack Destiny Bond, usually a characteristic attack of Ghost Pokemon. These slight differences have caused much confusion and debate among the Koffing fan circle.
Once it reaches level 35, it evolves into Weezing. I won't go into Weezing too much, but just know that it is basically the same deal with Weezing as it is Koffing, except bigger.
Koffing is best known as the Pokemon of choice for one of the TV series' main villains, James of Team Rocket. Koffing was in his possession for episodes 1-31, at which time it finally evolved into Weezing and continued to serve James in that form until, as far as I know, this day.
Mine's the coolest avatar cause he's round an' farty.
