Cheapest priced copy (Wilds of Eldraine Tokens): $0.35 at Card Kingdom, near mint, their price list as of .
Nightmare 
Creature — Nightmare Horse · */* · Rare · Collectors' Edition · #119
Flying (This creature can't be blocked except by creatures with flying or reach.)
Nightmare's power and toughness are each equal to the number of Swamps you control.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
31 printings · $0.35 to $3299.99 at Card Kingdom
4 rulings from Wizards of the Coast
- 1 April 2008If 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.
- 1 October 2009Nightmare’s power and toughness changes as the number of Swamps you control changes.
- 1 July 2013Nightmare’s ability counts all lands you control with the subtype Swamp, not just ones named Swamp.
- 1 July 2013The ability that defines Nightmare’s power and toughness works everywhere, not just on the battlefield.