Before you take your horse down the road do a pre-travel check of your truck and trailer to make sure everything is in working order. You don't want to get several miles down the road only to find out something is wrong. The goal is to get you and your horse safely to your destination.
Check the tire pressure on both the truck and trailer. Improper tire pressure can cause blow outs or cause the trailer to sway. Make sure you check the spares and that you have the proper tools to change a tire.
Check the lug nuts to make sure they are secure.
Make sure all the lights and turn signals work on both the truck and trailer.
Check all of the fluid levels on the truck and make to make sure they are normal.
Look inside the trailer to make sure there are no wasp or bee nests. You do not want to get your horse secured in the trailer only to find out there was a nest and now the wasps are mad this could be a disaster.
If it's a hot day you will want to open the roof vents. Open the vents in the front where they are facing forward and the back vents facing backwards for proper air flow. If you're traveling through climate extremes you might not want to open the vents because it could cause your horses to get sick.
If you open the windows make sure there is a guard on them. A screen is the best option because it prevents debris from coming in the window like cigarettes. This happened in 2011 where a cigarette was tossed out of another vehicle it started a fire in the trailer and six horses died.
Screens will also help prevent people from sticking their hands in the window to pet the horses. You can also put a fly mask on your horse to protect their eyes from foreign objects.
A word of caution you don't want to get too much air flow because it can cause respiratory problems from dried manure or sawdust flying around.
If you choose to feed your horse in the trailer only give them hay no grain and secure the haynet high so they won't get tangled up. Some horses get really stressed when traveling in the trailer which shuts down their digestion. Grain ferments easily and you don't want it setting in their stomach because it could lead to colic.
Make sure there are mats on the floor. The mats help the horse from slipping will cushion the ride and prevent the heat from coming through the flooring. Put shavings on top of the mats this helps to soak up the urine and will also help give them more cushion.
In a straight load trailer if you're only hauling one horse put them on the left (drivers) side. If your hauling two horses put the heavier one on the left (drivers) side. Doing this will help to balance the trailer because the road has a slight slope to the outside for moisture to runoff. It can also help if you happen to swerve and get your wheels off of the pavement.
Do this pre-check every time as it will make traveling with your horse a more pleasant experience.