Hydra Trainer - Modern Horizons 3

You may exert this creature as it attacks. When you do, target creature gets +X/+X until end of turn, where X is the number of counters on permanents you control. (An exerted creature won't untap during your next untap step.)
: Adapt 2. (If this creature has no +1/+1 counters on it, put two +1/+1 counters on it.)