Billie

Charge. If Billie moves at least 20 feet straight toward a target and then hits it with a ram attack on the same turn, the target takes an extra 5 (2d4) bludgeoning damage. If the target is a creature, it must succeed on a DC 13 Strength saving throw or be knocked prone.

Sure-Footed. Billie has advantage on Strength and Dexterity saving throws made against effects that would knock it prone.

Headstrong. Billie has a will of her own. At the start of combat Billies' rider needs to pass a DC 5 Animal Handling check for her to be considered a controlled mount (PHB 198). If the check fails she will act as an independent mount, even if she was summoned using the Finding Steed or Finding Greater Steed spells. In addition to this, she is immune to being charmed.

Ram. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 10 (3d4 + 3) bludgeoning damage.