There are two ways to cast creature cards with mutate:

• Cast the spell for its mana cost.
• Cast the spell for its mutate cost.

Casting a creature spell for its mutate cost also has a few requirements:

• You pay the mutate cost rather than the mana cost.
• A non-Human creature target you own currently on the battlefield.

Now, when a creature card cast for its mutate cost resolves, rather than just entering the battlefield, it merges with the target existing creature into one. You choose whether the card just cast goes on top of its target or underneath it. The resulting pile of cards & tokens all represent a single creature with all the characteristics of the top card, and it will also have the abilities of each card underneath.

Here are the answers to a few common questions when it comes to mutate:

Mutate FAQ

Can I use the mutate ability if I bring a creature card from my graveyard or library to the battlefield?

Mutate can only be used if you’re casting the spell. If the creature card were to enter the battlefield in another way, it just enters the battlefield.  

If I cast a creature for its mutate cost, do I also have to pay the mana cost?

No, if you cast a creature spell for its mutate cost on a valid target (a non-human creature you own), you only pay its mutate cost.

If I mutate a creature with another creature card, what is its name/creature type/color?

That creature will have all the characteristics of the top card, such as power and toughness, and it will also have the abilities of every card underneath. That means that the creature's name, converted mana cost, colors, and creature types are all determined by the top card.

If I cast a creature spell for its mutate cost to mutate an existing creature on the battlefield, does that trigger “enter the battlefield” effects?

No, the resolving mutating creature spell doesn't enter the battlefield. It simply makes the creature that was already on the battlefield change characteristics.

If I cast a mutate spell on a tapped creature, is it still tapped?

Yes, if that original creature was tapped, the new merged creature is tapped.

What happens when I mutate a creature that already has counters, Auras, or Equipment attached to it?

The new merged creature retains all of those counters, Auras, and Equipment.

What happens when a mutated creature leaves the battlefield?

If a mutated creature leaves the battlefield, all of its components go to the appropriate zone. So if it dies, each card ends up in the graveyard. Any ability that triggers "whenever a creature you control dies" or similar will trigger only once. Same with exile, your hand, or your library. If it's put into a specific position in your library, such as on the bottom or third from the top, you choose the relative order of the cards.

What happens if my mutating creature spell is countered?

While it's on the stack, a mutating creature spell is still a spell, so it can be countered. If it’s countered, it heads to the graveyard as you'd expect.

Now, if a mutating creature spell tries to resolve but can't because its target is illegal or has gone missing, the mutating creature spell will simply resolve and enter the battlefield. That said, although the spell resolves and the creature enters the battlefield, any "Whenever this creature mutates" abilities it has won't trigger.

Can I mutate a creature token?

Yes, tokens can mutate just like nontoken permanents can. For any merged creature, if a token is on top, the creature is a token. If a card is on top, the creature is a nontoken permanent.