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.)
Only a creature with a +1/+1 counter on it can be chosen as the first target of Mutant’s Prey.
2013-04-15 • wotc
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.