It lived in snowy areas for so long that its fire sac cooled off and atrophied. It now has an organ that generates cold instead. (Galarian Darumaka)
Powerful legs and jaws made it the apex predator of its time. Its own overhunting of its prey was what drove it to extinction. (Dracovish)
It was inside a meteorite that fell 20,000 years ago. There seems to be a connection between this Pokémon and the Dynamax phenomenon. (Eternatus)