Hidden Stag is a rare Enchantment card from Urza's Saga (USG), collector number 265. Current market price: $0.44, TCGplayer market prices via Scryfall.
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.