My first ever shiny was a Metapod in Pokemon Gold's Ilex Forest. I evolved it into the Pink Butterfree, and I liked how it reminded me of the one from the anime.
A funny story, the next shiny was an Unown in the same game. I was riding in a car, and I actually ran from it, not knowing that it's color was different (they look sorta similar), and I was looking in another direction while I encountered it. The next second, I realized that the Unown was slightly blue. But it was too late. FML. >.>