Navin
MALDREAD
What about Gligar in Sinnoh, I remember thinking along the same lines that he wouldn't catch any older guys that could evolve into new ones, yet he did.
Gliscor was an entirely new Pokemon though compared to Mega/Alolan variants being modifications of preexisting Pokemon, but I see your point. I'm all for Alolan Sandslash personally, even if I don't see it necessarily happening.