Historical Rules · 8 July 2022
Rule 702.140c
Official source text from the 8 July 2022 Rules release
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 724, “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.