Rock Hydra | Magic: The Gathering
Formats Rock Hydra is Legal in
Official Oracle Text for Rock Hydra
For each 1 damage that would be dealt to Rock Hydra, if it has a +1/+1 counter on it, remove a +1/+1 counter from it and prevent that 1 damage.
: Prevent the next 1 damage that would be dealt to Rock Hydra this turn.
: Put a +1/+1 counter on Rock Hydra. Activate this ability only during your upkeep.
Official Rulings for Rock Hydra
10/4/2004 : Once on the battlefield, the X is considered to be zero when calculating its mana cost.
8/1/2008 : You 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.
8/1/2008 : If 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.