Mistcutter Hydra

This spell can't be countered.
Haste, protection from blue
Mistcutter Hydra enters with X +1/+1 counters on it.

Asceticism

Creatures you control have hexproof.
{1}{G}: Regenerate target creature. (The next time it would be destroyed this turn, instead tap it, remove it from combat, and heal all damage on it.)

Kalonian Hydra

Trample
Kalonian Hydra enters with four +1/+1 counters on it.
Whenever Kalonian Hydra attacks, double the number of +1/+1 counters on each creature you control.

Blossoming Defense

Target creature you control gets +2/+2 and gains hexproof until end of turn.

Steelbane Hydra

Steelbane Hydra enters with X +1/+1 counters on it.
{2}{G}, Remove a +1/+1 counter from Steelbane Hydra: Destroy target artifact or enchantment.

Whisperer of the Wilds

{T}: Add {G}.
Ferocious — {T}: Add {G}{G}. Activate only if you control a creature with power 4 or greater.

Primordial Hydra

Primordial Hydra enters with X +1/+1 counters on it.
At the beginning of your upkeep, double the number of +1/+1 counters on Primordial Hydra.
Primordial Hydra has trample as long as it has ten or more +1/+1 counters on it.

Nissa, Who Shakes the World

Whenever you tap a Forest for mana, add an additional {G}.
+1: Put three +1/+1 counters on up to one target noncreature land you control. Untap it. It becomes a 0/0 Elemental creature with vigilance and haste that's still a land.
−8: You get an emblem with "Lands you control have indestructible." Search your library for any number of Forest cards, put them onto the battlefield tapped, then shuffle.

Forest

Gargos, Vicious Watcher

Vigilance
Hydra spells you cast cost {4} 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.

Harmonize

Prey Upon

Target creature you control fights target creature you don't control. (Each deals damage equal to its power to the other.)

Nissa's Pilgrimage

Search your library for up to two basic Forest cards, reveal those cards, and put one onto the battlefield tapped and the rest into your hand. Then shuffle.
Spell mastery — If there are two or more instant and/or sorcery cards in your graveyard, search your library for up to three basic Forest cards instead of two.