Rock Hydra — ruling 1

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.

Rock Hydra — ruling 2

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.

Rock Hydra — ruling 3

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

Rock Hydra — ruling 4

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

Rock Hydra : règles, fonctionnement et interactions (MTG)

Rock Hydra is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

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. : Prevent the next 1 damage that would be dealt to this creature this turn. : Put a +1/+1 counter on this creature. Activate only during your upkeep.

How it works : Rock Hydra

Original English text — Wizards of the Coast / Scryfall

#1August 1, 2008· Wizards of the Coast
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.
No other card is named in this ruling.
#2August 1, 2008· Wizards of the Coast
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.
No other card is named in this ruling.
#3October 4, 2004· Wizards of the Coast
If put onto the battlefield without casting it from your hand, X will be zero.
No other card is named in this ruling.
#4October 4, 2004· Wizards of the Coast
Once on the battlefield, the X is considered to be zero when calculating its mana cost.
No other card is named in this ruling.