Calling All Hydra Heads

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 fights up to one target creature you don't control.

Hydra

Hydroid Krasis

When you cast this spell, you gain half X life and draw half X cards. Round down each time. Flying, trample This creature enters with X +1/+1 counters on it.

Primordial Hydra

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

Unbound Flourishing

Whenever you cast a permanent spell with a mana cost that contains {X}, double the value of X. Whenever you cast an instant or sorcery spell or activate an ability, if that spell's mana cost or that ability's activation cost contains {X}, copy that spell or ability. You may choose new targets for the copy.

Zaxara, the Exemplary

Deathtouch {T}: Add two mana of any one color. Whenever you cast a spell with {X} in its mana cost, create a 0/0 green Hydra creature token, then put X +1/+1 counters on it.

Shadow Sliver (foil)

All Sliver creatures have shadow. (They can block or be blocked by only creatures with shadow.)