Big Mac has all prominent features of a Clydesdale but the largest. He's not palomino, so he could be a part Walking Horse or Quarter Horse. I'm not sure if he is a fast runner, because that would suggest that he was Quarter. More graceful and delicate would be Walking Horse.
I HATED THIS EPISODE!!!!!!! I thought Twi would be the Princess of Stars NOT Friendship. At least there was a Luna/Celestia/Cadence song. They'd really needed to do that for the entire show.