Liability follows the tow. Damage does not.
If a trailer you are towing causes injury or damage, your auto liability generally responds. If the trailer itself is wrecked, stolen or burned, a personal auto policy usually pays nothing for it. That split surprises people with an expensive trailer and a cheap truck.
What is inside the trailer is another question again
Tack, tools, equipment and animals in transit are not auto property. They fall to a homeowners policy with its own limits, to an inland marine or equipment policy, or to nothing at all. Horses in transit in particular are not covered by either the auto or the homeowners form.
Heavier loads change the liability math
A loaded trailer stops longer and does more damage when it does not stop. The exposure is larger than the vehicle alone suggests, and the liability limit chosen for a pickup was probably not chosen with three tons behind it in mind.