But you may want to think about something more than "Care for the horse as if he/she were your own." That might be because it turns out that you'd do things just a tad differently from your BFF, or you'd like to spare them some tough decisions. After all, the trustee is an executor of sorts: He/she is simply supposed to carry out what the grantor (the person who formed the trust) wanted. Make that clear *and feasible*; God knows some unforeseen circumstances will arise to muddy the waters.
In my case, that's because I have unusually high standards about euthanasia. E. g. most people wait for the standard "can't keep weight on, gets that far a way look," while I'd pull the plug at "can't lie down anymore and is falling asleep on his feet" or "is getting chased by the pasture mates and looks worried or beat up all the time."
Also, I think my trustees (great friends and very good horse people) might feel guilty about not going to the ends of the earth for their dead buddy's horse. For an oldie, I'd do "no vet treatments that can't be done on the farm." So I'd specify that so that my buds didn't have to agonize.
Do you have a retirement farm (or 3) that you'd like the horse sent to?
Or what if you have a great young horse that a friend might like to flip? Do you have an opinion about that?
The beauty of a trust is that you can specify any details you'd like, so long as they are legal. The beauty of a good trust attorney is that he/she can advise you on what conditions are likely to be enforceable.