Rock Hydra

Rock Hydra enters 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.
{R}: Prevent the next 1 damage that would be dealt to Rock Hydra this turn.
{R}{R}{R}: Put a +1/+1 counter on Rock Hydra. Activate only during your upkeep.

30th Anniversary Edition

Masters Edition IV

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.
{R}: Prevent the next 1 damage that would be dealt to Rock Hydra this turn.
{R}{R}{R}: Put a +1/+1 counter on Rock Hydra. Activate this ability only during your upkeep.

Summer Magic / Edgar

Revised Edition

Put X +1/+1 counters (heads) on Hydra. Each point of damage Hydra suffers kills one head unless controller spends o
R per head. During controller's upkeep, new heads may be grown for o
Ro
Ro
R apiece.

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

Once on the battlefield, the X is considered to be zero when calculating its mana cost.

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.

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.