Marsh Mutant

Hexproof (This creature can't be the target of spells or abilities your opponents control.)
Evolve (Whenever a creature enters the battlefield under your control, if that creature has greater power or toughness than this creature, put a +1/+1 counter on this creature.)
Whenever a +1/+1 counter is put on Marsh Mutant, target creature gains flying until end of turn.