Whenever Wild Beastmaster attacks, each other creature you control gets +X/+X until end of turn, where X is Wild Beastmaster's power.
10/1/2012 The value of X is determined when the triggered ability resolves. If Wild Beastmaster is no longer on the battlefield at that time, use its last known power to determine the value of X.
7/14/2017 If this creature’s power is negative as its ability resolves, X is considered to be 0. (This is a change from previous rulings.)