Congrats!
The first time I caught a Shiny that was neither a distribution event, in-game event (ie Gyarados) or deliberately hunted/increased odds (ie chain fishing) was last summer when I stumbled upon a shiny Drowzee in SoulSilver (incidentally I was simply looking for a Hypno for my living dex and didn't care that it was shiny). I also recently stumbled upon a shiny Nuzleaf in Friend Safari, looking for a completely different Pokemon altogether, but I know there are increased odds there.
darkx
Games beaten in 2023 - 4; Most recent - Pushmo