Unless having mini horses aboard upsets the tongue weight significantly, you should be well served by a recent model F150.

Do get a brake controller installed and use it regardless of the tow vehicle.... 1/2 ton or full 1 ton truck

I feel that an properly equipped F150 with the proper hitch, and a good brake controller should do a good job towing that steel two horse trailer. If it's an older used trailer, you may want to have the wheel bearings inspected and repacked with grease, along with having the brakes inspected and manually adjusted before hauling with it. Check the build date on the tires, if they are five years old or older I would suggest you replace them. They may "look brand new", but if they're old, they are dry rotting and cracking. Load it up with horses and gear, hit the highway and you'll inevitably going to have a blowout and or possibly an accident. If this trailer has mats, you may want to pull them out, wash and inspect the floor boards for rot, cracks and or broken boards and trailer frame structure..... If you'll check theses things before hand, then your mother can rest assured you've done everything you possibly can to insure safe travel for you, your husband and the horses that she would rather you didn't have.... Good luck with your new toys!!!