When Craterhoof Behemoth enters the battlefield, creatures you control gain trample and get +X/+X until end of turn, where X is the number of creatures you control.
3/14/2017 The value of X is determined only as Craterhoof Behemoth’s triggered ability resolves, potentially including Craterhoof Behemoth itself. It won’t change later in the turn if the number of creatures you control changes.
3/14/2017 Craterhoof Behemoth’s triggered ability affects only creatures you control at the time it resolves. It won’t affect creatures that come under your control later in the turn.