Hidden Stag 
Enchantment
Whenever an opponent plays a land, if this permanent is an enchantment, it becomes a 3/2 Elk Beast creature. Whenever you play a land, if this permanent is a creature, it becomes an enchantment.
Legality
standardNOT LEGALpioneerNOT LEGALmodernNOT LEGALlegacyLEGALvintageLEGALcommanderLEGAL