Creature — Nightmare Horse */*, 5B (6)
Rare
Flying
Nightmare's power and toughness are each equal to the number of Swamps you control.
The Nightmare arises from its lair in the swamps. As the poisoned land spreads, so does the Nightmare's rage and terrifying strength.
Illus. Melissa Benson

4/1/2008: If you control 0 swamps, then the Nightmare has 0 toughness and will be put into its owner's graveyard as a state-based action right before the next player gains priority.
10/1/2009: Nightmare's power and toughness changes as the number of Swamps you control changes.
Legal in Vintage (Type 1)
Legal in Legacy (Type 1.5)
Legal in Classic (MTGO)
Legal in Commander
Legal in Modern