Sawtusk Demolisher

• WOTC Official Reminder Textofficial rules
  Mutate — If you cast this spell for its mutate cost, put it over or under target non-Human creature you own. They mutate into the creature on top plus all abilities from under it.

• WWizards of the Coast2020-04-17
  Official Ruling
  If the target noncreature permanent is an illegal target by the time Sawtusk Demolisher’s last ability tries to resolve, the ability won’t resolve. No player will create a Beast token. If the target is legal but not destroyed (most likely because it has indestructible), its controller does create a Beast token.

Rulings via Scryfall