Rock Hydra

Creature — Hydra    (CMC:2)

Rock Hydra enters the battlefield with X +1/+1 counters on it. 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.

133 ME4 • ENJeff A. Menges

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Legal in: Legacy,Vintage,Commander

Oracle Text (click to copy):

10/4/2004
If put onto the battlefield without casting it from your hand, X will be zero.
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.