I suggest for a 2-4 horse trailer, at least a 1/2 ton full sized truck. (this includes full size suv's as Suburbans/Tahoes(chevy/gmc) and Excursions/Expeditions(ford).)
I would also have trailer brakes for anything larger than a 2 horse trailer.
I air on the side of caution when hauling live animals. I have hauled a heavy car trailer with our Jeep Cherokee....but as I said earlier, a heavy trailer will push around a smaller vehicle.
What kind of trailer do you have and how many horses are you transporting??? That would help us make better recomendations.