For a horse trailer, because of the weight, I am thinking you are going to want a Torklift Super Hitch and Super Hitch extension.
I don't haul that heavy with my Northern Lite as I just haul an ATV trailer. I do know somebody who also has a Northern Lite and she hauls her two mules all over the place and she has a Super Hitch and Super Hitch extension on her Ford Super Duty dually.
2017 Ford F350 XLT Premium CCSB 4x4 6.2 gas 3.73 rear end, 4226 lbs payload
2017 Northern Lite QC 8.11 SE
Torklift tie downs, Torklift Fast Guns, Torklift Upper Stableloads, Airlift 5000 Ultimate air bags, Airlift WirelessAIR onboard compressor system