Vigilance

Hydra spells you cast cost less to cast.

Whenever a creature you control becomes the target of a spell, Gargos, Vicious Watcher fights up to one target creature you don't control.

Gyrus, Waker of Corpses enters with a number of +1/+1 counters on it equal to the amount of mana spent to cast it.

Whenever Gyrus attacks, you may exile target creature card with lesser power from your graveyard. If you do, create a token that's a copy of that card and that's tapped and attacking. Exile the token at end of combat.

Reach, lifelink

Whenever Polukranos, Engine of Ruin or another nontoken Hydra you control dies, create a 3/3 green and white Phyrexian Hydra creature token with reach and a 3/3 green and white Phyrexian Hydra creature token with lifelink.

: Monstrosity X. (If this creature isn't monstrous, put X +1/+1 counters on it and it becomes monstrous.)

When Polukranos, World Eater becomes monstrous, it deals X damage divided as you choose among any number of target creatures your opponents control. Each of those creatures deals damage equal to its power to Polukranos.

Flying

The Goose Mother enters with X +1/+1 counters on it.

When The Goose Mother enters, create half X Food tokens, rounded up.

Whenever The Goose Mother attacks, you may sacrifice a Food. If you do, draw a card.

Deathtouch

: Add two mana of any one color.

Whenever you cast a spell with in its mana cost, create a 0/0 green Hydra creature token, then put X +1/+1 counters on it.