Cheapest priced copy (Revised Edition): $1.99 at Card Kingdom, near mint, their price list as of .
Rock Hydra 
Creature — Hydra · 0/0 · Rare · Intl. Collectors' Edition · #172
This creature enters with X +1/+1 counters on it.
For each 1 damage that would be dealt to this creature, if it has a +1/+1 counter on it, remove a +1/+1 counter from it and prevent that 1 damage.
{R}: Prevent the next 1 damage that would be dealt to this creature this turn.
{R}{R}{R}: Put a +1/+1 counter on this creature. Activate only during your upkeep.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
10 printings · $1.99 to $1499.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Unlimited Edition | 172 | Rare | $37.99 | — |
| 30th Anniversary Edition | 167 | Rare | $18.99 | — |
| 30th Anniversary Edition | 464 | Rare | $249.99 | — |
| Revised Edition | 173 | Rare | $1.99 | — |
| Collectors' Edition | 172 | Rare | $8.99Out of stock | — |
| Intl. Collectors' Edition | 172 | Rare | $13.99 | — |
| Foreign Black Border | 173 | Rare | — | — |
| Limited Edition Alpha | 171 | Rare | $1499.99 | — |
| Limited Edition Beta | 172 | Rare | $349.99 | — |
| Summer Magic / Edgar | 173 | Rare | — | — |
4 rulings from Wizards of the Coast
- 4 October 2004If put onto the battlefield without casting it from your hand, X will be zero.
- 4 October 2004Once on the battlefield, the X is considered to be zero when calculating its mana cost.
- 1 August 2008If damage that would be dealt to Rock Hydra can't be prevented, you still remove a +1/+1 counter from it for each 1 damage dealt.
- 1 August 2008You can activate the activated ability that prevents damage any time, even if no source seems likely to deal damage to you in the near future. It will prevent the next 1 damage that would be dealt this turn, even if that turns out to be much later in the turn.