Remember the Red Gyarados from the Lake of Rage?

I'm a little unclear on what the explanation was there. Was he a shiny? Because the story was that Team Rocket had just force-evolved a Magikarp and the result was this Gyarados whose scales hadn't even had the time to change color to blue.

But even if he was a shiny, was he, like, the first shiny ever? Because everyone was freaking out like they'd never seen a Pokemon of an alternate color before.

I'm sure somebody can weigh in and set this straight for me.

There's fundamentally not much of a difference between a shiny and just any other Pokémon of alternate color (from an in-world, faux-biological standpoint).

They're both mutations (and non hereditary mutations at that) but I guess shinies are consistent in a way that just randomly having a different color wouldn't be.

I wouldn't say it was the first shiny so much as it was the first shiny to be widely seen and broadcasted. The out-of-world explanation is that they wanted GF to draw attention to their shiny (ba dumm tss) new mechanic and worked it into the story, but since you can get shinies before the Red Gyarados event they were presumably always there, you just never lucked into one in Gen I / people didn't really see them before.

Like Pokémon eggs - presumably Pokémon always bred using eggs before Mr. Pokémon / the Daycare Couple saw one, humans just didn't really see the eggs.