This will depend on several factors: the type of trailer you have, the number of horses, and their weight. The general rule of thumb is to put the heavier horse on the left or the driver’s side of your trailer. Most roads are slightly higher in the middle than on the sides. Having your heavier load on the left side will give your trailer better balance.  If you have a straight load trailer, load the heavier horse on the left side. If you have a slant load trailer, put the heavier horse in the front. A single horse should be put in the front of a slant load trailer and on the driver’s side in a straight load trailer.