towing a horse trailer
Much depends on the weight and make of the trailer, (and your horses, mine were arabs, much lighter than quarter horses) some trailers are more stable and aerodynamic than others. Some should never be on the road. Look up the Brenderup brand for example. The top of the line for two horse trailers. Made specifically for hauling horses in Europe on the autoban with smaller vehicles. In your case for now, make sure your towing vehicle always weighs more than your horses and trailer. Even if you have to haul some hay bales in the truck. Also when you haul only one horse, load it on the left side, due to the way roads are banked and curved it is safer. Any time your trailer weighs more than your hauling vehicle you are in danger. You might also look into installing sway bars, and as stated above, err on the side of caution. The size engine you are using will mean slow hill climbs and not a lot of guts on dirt roads.
I have hauled horses for 50 years with every conceivable configuration of truck and trailer. When you are on the highway with your four legged friends, for the sake of their lives and yours and those all around you on the road it is best not to go under powered. A bigger engine would serve you longer and be more efficient in the end, the smaller less powerful one will wear out sooner.