Ambusher. The adult allosaurus has advantage on attack rolls against any creature it has surprised.

Pounce. When the adult allosaurus makes a claw attack against a surprised target, the target takes an additional 9 (2d8) slashing damage and must succeed on a DC 14 Strength saving throw or be knocked prone.

Bite. Melee Attack Roll: +6, reach 5 ft. Hit: 15 (2d10 + 4) Piercing damage.

Claws. Melee Attack Roll: +6, reach 5 ft. Hit: 8 (1d8 + 4) Slashing damage. If the target is a Large or smaller creature and the allosaurus moved 30+ feet straight toward it immediately before the hit, the target has the Prone condition, and the allosaurus can make one Bite attack against it.

If the target is prone, the allosaurus can make one bite attack against it as a bonus action.