You are able to increase one of your ability scores by 1, and another ability score by 2. Alternatively, you can increase three different ability scores by 1 each.

Horses are beasts of burden who are commonly used to haul cargo and passengers over long distances at relatively fast speeds. Therefore, Awakened Horses retain their feral relatives' talent for going real fast and have 40 feet of movement per turn in combat as opposed to the traditional 30.

This seems fairly obvious but horses aren't humans. Even Awakened Horses, with the intellect of humans or even greater, still look and smell and walk on four legs like a normal horse does. Because of this, the creature type of Awakened Horses counts as beast and not humanoid for the purpose of abilities and features that target specific creature types, and their default size is large instead of medium. Awakened Horses are also naturally able to speak with animals without magical assistance of any kind.

Awakened Horses can use their hooves to potentially deal more damage in unarmed attacks. Specifically, unarmed attacks from Awakened Horses deal 1d6 bludgeoning damage plus their Strength modifier. However, hooves also make a distinct noise when walking so Awakened Horses have disadvantage on Stealth checks made to sneak around undetected.

Awakened Horses may need special accommodations for armor given that they have the bodies of horses and not humanoids. However, they are naturally a bit tougher given the fact that they're massive beasts of burden, so they have a +1 bonus to their armored class while not wearing armor.

Awakened Horses still look and smell and walk like normal horses, etc etc. Therefore, it's easier for them to come across as unthreatening and incapable, especially when pretending to be normal horses with no capacity for speech and spellcasting and all that fancy stuff smart humanoids can do. Awakened Horses have advantage on Deception checks to appear as unthreatening to others.

Awakened Horses still have all the features of normal horses, therefore medium creatures (and smaller) can ride on their backs as if they were mounts. All normal rules for mounts apply in this situation, such as mounted combat.