As Ann Romney immersed herself in the elite world of riding over the last dozen years, she relied on Jan Ebeling as a trusted tutor and horse scout. In her, he found a deep-pocketed patron.

A taskmaster, Mr. Ebeling pushed Mrs. Romney to excel in high-level amateur shows. He escorted her on horse-buying expeditions to Europe. She shares ownership of the Oldenburg mare he dreams of riding in the Olympic Games this summer. Mrs. Romney and her husband, Mitt, even floated a loan — $250,000 to $500,000, according to financial records — to Mr. Ebeling and his wife for the horse farm they run in California, where the Romneys use a Mediterranean-style guesthouse as a getaway.