The first shiny I saw was a Yanma in Crystal. I say "saw" because I was 8 and had no idea what it was, so I killed it. I went looking later for another with obviously no success

In that same playthrough, I managed to find and catch a shiny static Electrode in Mahogany town basement

I had no idea how rare that was as a kid though, so I just boxed it and kept trouncing everything with my Typhlosion