פירוש על עבודה זרה 31:5

The rabbis have a notion that carrying a person is not considered forbidden labor on Shabbat because a “live person carries himself.” Since a horse is used for riding, Ben Batera says one may sell a horse to a non-Jew because the horse does not perform labor forbidden on Shabbat from the Torah.
Rabbi [Judah Hanasi] forbids selling horses for two reasons. First of all, they are weapons. Second of all, they are considered large animals.