There are a couple versions of how Rat Fink came about. The one most often told is the version where Big Daddy is in a restaurant, and draws on a napkin, a character to show what Mickey Mouse's father might have looked like.
Origin of Rat Fink
You can find Rat Fink on t-shirts, key chains, pins, models, latex toys, patches, decals, plus more items are coming out every year.
1990 Rat Fink Toy Commercial