Scot Gillies has a good idea of the kind of horse people who will fit in at Gryffindor Farm, the small barn he helps manage in Lexington, Ky. So his advertisements for new boarders spell it out in detail: Owners must be "laid-back," "happy" and above all, "drama-free."

Mr. Gillies, a marketing consultant by day and a horse owner himself, says that during his 14 years in the equestrian world, "I've seen the full range of drama that is associated with horse people."

...