Koffing = Coughing
Weezing = Sneezing
Therefore, Sneasel was intended to be a split evolution to Koffing. This explains why Koffing's index number is 55 and Weezing's is the far off 143, while a Missingno with the index number 56, when traded to Gold or Silver, becomes a Sneasel.
http://bulbapedia.bulbagarden.net/wiki/List_of_Pok%C3%A9mon_by_index_number_%28Generation_I%29This is also why Sneasel can learn the poison move Toxic via TM, a trait that is completely noteworthy and definitely doesn't apply to the vast majority of Pokemon.
HOW DEEP DOES THE RABBIT HOLE GO?
