Migratory Greathorn is a common Creature — Beast card from Magic Online Promos (PRM), collector number 80981.
Mutate {2}{G} (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.) Whenever this creature mutates, search your library for a basic land card, put it onto the battlefield tapped, then shuffle.