Horses are commonly parasitized by bot fly larvae (Gasterophilus). Throughout the summer, bot flies lay their eggs on the hair shafts of horses at pasture. These small yellow bot fly eggs can easily be seen on dark-colored horses. The flies selectively lay their eggs on the hair coat of the fore quarters from where the horse ingests the eggs during grooming (licking) behavior. These eggs develop into larval stages in the horse's stomach and may, in sufficiently large numbers, lead to gastric irritation and mild colic. Mature larvae are passed out in feces to pupate on the ground. Large numbers of larvae in the rectum (temporary attachment) have been reported to cause straining.
For more information, please read the article Equine Gastrointestinal Parasites