Haste
When this creature enters, creatures you control gain trample and get +X/+X until end of turn, where X is the number of creatures you control.
Craterhoof Behemoth’s triggered ability affects only creatures you control at the time it resolves. Creatures you begin to control later in the turn won’t gain trample and get +X/+X.
The value of X is calculated only once, as Craterhoof Behemoth’s last ability resolves.