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

Ideal Mount. When you use this creature as a mount, you may choose to use it as an Ideal Mount.  When atop an Ideal Mount, any attacks, spells, or abilities that would effect it effect you instead.  Your Hit Point maximum is increased by a number of HP equal to the Sumpter's Total hit points (10).  It does not get its own turn in combat, instead lending its speed and strength to you.  Your movement speed becomes its movement speed (40ft).  If you can take the dash action as a bonus action, it cannot take the dash action as a bonus action while Sumpter is your Ideal Mount.  Your AC becomes equal to whichever AC is lower, yours or the Sumpter's (10).  You also gain a Charge Attack when using your Ideal Mount.  If you dismount for any reason, your Maximum HP returns to normal.  If your HP becomes 0 while you have an Ideal Mount, you automatically dismount and both you and Sumpter begin to make death saving throws.

Charge Attack. Whenever you move at least 10ft in a straight line at an opponent before immediately taking the attack action, you may replace your first attack with a Charged Attack.  When you do, you gain a +4 bonus to the attack roll and +1d6 to the weapon's damage.  If you have the Extra Attack feature, this bonus does not apply to any future attacks unless you have moved at least 10ft in a straight line towards the enemy before making the attack.

Hooves. Melee Weapon Attack. 5ft +6 to hit. 2d4 +4 (8) [bludgeoning].