Quote from ReapThaWhirlwind »
Yes it does, it simply replaces the initial engagement dynamic with a bypass to the resolved effect.
702.139a 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).
702.139b 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.
702.139c 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 721, “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.
Quote from ReapThaWhirlwind »Why not just,
"return it to the battlefield mutated with target creature"?
This could include a 'in any way you choose' clause, but I would question how necessary that is.
Quote from Kryptnyt »Grusilda's Tools 3
5, T, Sacrifice Grusilda's Tools; Exile target creature, then return it to the battlefield as though it were cast with a mutate cost targeting another target creature you control.
Removal spell and goofy combo enabler. Probably a rules nightmare. Didn't include "non-Human" anywhere here, partially because I don't like the "non-Human" part of Mutate, and partially because Grusilda wouldn't either.