Migratory Greathorn

Mutate appears on some creature cards. It represents a static ability that functions while the spell with mutate is on the stack. “Mutate [cost]” means “You may pay [cost] rather than pay this spell’s mana cost. If you do, it becomes a mutating creature spell and targets a non-Human creature with the same owner as this spell.” Casting a spell using its mutate ability follows the rules for paying alternative costs (see 601.2b and 601.2f–h).

As a mutating creature spell begins resolving, if its target is illegal, it ceases to be a mutating creature spell and continues resolving as a creature spell and will be put onto the battlefield under the control of the spell’s controller.

As a mutating creature spell resolves, if its target is legal, it doesn’t enter the battlefield. Rather, it merges with the target creature and becomes one object represented by more than one card or token (see rule 727, “Merging with Permanents”). The spell’s controller chooses whether the spell is put on top of the creature or on the bottom. The resulting permanent is a mutated permanent.

An ability that triggers whenever a creature mutates triggers when a spell merges with a creature as a result of a resolving mutating creature spell.

A mutated permanent has all abilities of each card and token that represents it. Its other characteristics are derived from the topmost card or token.

Any effect that refers to or modifies the mutating creature spell refers to or modifies the mutated permanent it merges with as it resolves.