You know, the way they put bugs in them? ... Anyway...
Pidgey was never my favourite pokemon, but it was always in my party for a long time. This is because it grew so fast. I usually had a higher-level Pidgeotto than my starter until Vermillion City. It was also my Flyer.

Actually, now that I think of it, how DID that guy with the level 20 Butterfree beat me? Maybe my Pidgeotto fainted earlier and I didn't go back and heal because I was almost there?