Capricopian
{X}{G}
Creature -- Goat Hydra
0/0
~this~ enters with X +1/+1 counters on it.
{2}: Put a +1/+1 counter on ~this~, then you may reselect which player ~this~ is attacking. Only the player ~this~ is attacking may activate this ability and only during the declare attackers step. (It can't attack its controller.)