Creature — Hydra
Trample Kalonian Hydra enters the battlefield with four +1/+1 counters on it. Whenever Kalonian Hydra attacks, double the number of +1/+1 counters on each creature you control.
To double the number of +1/+1 counters on a creature, determine how many +1/+1 counters are on the creature and put that many more on it. Effects that interact with counters (such as the one created by Corpsejack Menace’s ability) may change the number of counters ultimately put on the creature.