פירוש על עבודה זרה 31:6

The prohibition of selling weapons applies to a horse, for a horse can kill by trampling. Indeed, it seems quite clear that horses are used as weapons. But the prohibition of selling large cattle is more difficult to explain, for as I said above, the rabbis do not consider carrying a human to be prohibited by the Torah. R. Yohanan answers that when the animal grows old it will be made to work in a mill, which is considered work.