On that same cartridge I found a shiny Whismur while looking for a Whismur for my in-game team and managed to catch that too.
haha same happened here :D ... sort of. My in-game team plan included Fearow, and I was training a Fearow I had caught earlier when I ran across a shiny Spearow, which promptly became a team member and is now my highest-level Pokémon in FireRed.