Target creature you control with a +1/+1 counter on it fights target creature an opponent controls. (Each deals damage equal to its power to the other.)
If one of the targets is illegal when Mutant’s Prey tries to resolve (for example, if the first target creature no longer has a +1/+1 counter on it), neither creature will deal or be dealt damage.
Only a creature with a +1/+1 counter on it can be chosen as the first target of Mutant’s Prey.