A frozen Mamoswine was dug from ice dating back 10,000 years. This Pokémon has been around a long, long, long time.
A frozen one was dug up from soil dating back 10,000 years. It woke up, to much amazement.
This species reached its zenith during the period known as the ice age. I suspect that Hisui’s frigid climate is in harmony with Mamoswine’s constitution, thus awakening hidden potential.