Towering Stature. The brontosaurus is large enough that creatures that are Medium-sized or smaller can enter its space and end their turns there. If a creature ends its turn in the brontosaurus's space, the brontosaurus can make a stomp attack against that creature as a reaction.

Long Neck. The brontosaurus' long neck gives it a good vantage point. It has advantage on Wisdom (Perception) checks to perceive creatures that are within 500 feet of it and are Medium or larger, but disadvantage on creatures that are small or smaller.

Sure-Footed. The brontosaurus has advantage on Strength and Dexterity saving throws made against effects that would knock it prone but cannot take the Dash or Disengage action.

Beast of Burden. The brontosaurus is considered to be a huge animal for the purpose of determining its carrying capacity.

Combative Mount. A trained dinosaur is proficient with light, medium, and heavy armor.

Stomp. The brontosaurus brings down its foot in a 10 foot square within 5 feet. Each creature in that space must make a DC 14 Dexterity saving throw, taking 17 (2d12 + 5) bludgeoning damage and 12 (2d12) thunder damage on a failed save, or half as much damage on a successful one. Creatures who fail this save by 5 or more are knocked prone.

Tail. Melee Weapon Attack: +8 to hit, reach 20 ft., one target. Hit: 35 (6d8 + 5) bludgeoning damage.