They are mineral accumulations around a foreign object that form round, triangular, or flat stones inside the bowel of a horse, usually over the course of several years.
They originate in the large colon and can lead to colic.
Refer to this Q&A by the UC Davis School of Veterinary Medicine.
The stones in the following photos were passed by a mare earlier this week, two on June 20 and two on June 21.
Symptoms began on June 19, including loss of appetite, restlessness, laying down.
The incident was preceded by diarrhea and appears to be resolving but is not over.
Enteroliths are associated with diets that are high in magnesium and protein, present in alfalfa, a staple for wild horses in off-range corrals.
The mare is a registered Paint and is 12 years old.
Western Horse Watchers Association 