Minimor and others make a good point, and one that you will find STRONGLY repeated on the truck/trailer forum(filled w/ folks who do a LOT of trailering of horses, over on HorseTrailer World)...that it is ALWAYS better to have 'too much' towing vehicle rather than too little...especially from the standpoint of being SAFELY able to stop the towed weight!
Years ago, I pulled w/ half-ton pickups(F-150s; in fact, my FIRST Ford Pickup was an F 100, back in about 1960!I had a two horse Straight load steel Stidham, that weighed about 2200 lbs including the old-style 'woven rubber' mats.Later, I had a couple more half-tons, BUT, with the BIG V-8 engines--our first SuperCab, a '75, had a 390...still w/the Stidham, and although they slowed a bit uphill w/ two 'big' horses, they were OK(the Stidham didn't have trailer brakes; NEVER was that an issue.) But, IMO, the newer half-tons aren't really up to PROPERLY towing much. I now, and for the past 11 years, have had a 3/4 ton SuperDuty.
I have a friend who has a Tundra,a V-6 to be sure, but I drove it pulling her single Brenderup(a super LIGHTWEIGHT horse trailer) w/ a couple of minis in it, and it faded NOTICABLY on pretty minimal hills, even with that minimal load.
My advice would be to upgrade your TRUCK to a 3/4 ton with a suitably adequate engine if you are going to continue to haul horses. I like the adage: "better to have it and not need it, than to need it and not have it"...told me by my great riding instructor many years ago!
Margo