Here is a link to fact sheet on eating feces in Dr Bray’s Corner which provides the facts and myths of coprophagy. Be sure there is adequate fiber provided by long stem grass hay. If your pony is consuming hay quickly or pushed off by paddock mates your only choice is to isolate him from the group. It’s rare for a horse to eat their own, but if so, you may want to distributed the hay feeding stations to force movement and slow the rate of hay consumption. That’s more of a long shot.
Coprophagy is not a nutritional deficiency but is behavioral. Some have suggested that the condition is similar to compulsive disorders.