Cheapest priced copy (RNA Guild Kit): $0.49 at Card Kingdom, near mint, their price list as of .
Carnival Hellsteed 
Creature — Nightmare Horse · 5/4 · Rare · RNA Guild Kit · #62
First strike, haste
Unleash (You may have this creature enter with a +1/+1 counter on it. It can't block as long as it has a +1/+1 counter on it.)
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
3 printings · $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| RNA Guild Kit | 62 | Rare | $0.49 | — |
| Return to Ravnica Promos | 147★ | Rare | — | $0.49 |
| Return to Ravnica | 147 | Rare | $0.49 | $0.49 |
4 rulings from Wizards of the Coast
- 15 April 2013You make the choice to have the creature with unleash enter the battlefield with a +1/+1 counter or not as it’s entering the battlefield. At that point, it’s too late for a player to respond to the creature spell by trying to counter it, for example.
- 15 April 2013Putting a +1/+1 counter on a creature with unleash that’s already blocking won’t remove it from combat. It will continue to block.
- 15 April 2013The unleash ability applies no matter where the creature is entering the battlefield from.
- 15 April 2013A creature with unleash can’t block if it has any +1/+1 counter on it, not just one put on it by the unleash ability.