I think you might need to re-weigh your trailer. The Dakota 2H slants w/ tack I was looking at earlier this year all weighted under 3,000lbs, NOT 7k! If your trailer is in fact hitting at 7k, you do need a bigger truck, but if you Dakota is more 'normal' weight, the F-150 should be fine. Inventory
The GVW (7,000) is the max total weight that can/should be loaded, NOT the empty weight of the trailer. The trailer by itself weighs in at 2,650. So that, plus two 1100lb horses and tack should come in about 5,000lbs total, plus whatever crap you accumulate in the tack room. That is well within the safe towing capacity of an F150. You'd be fine.