Growing up, I was an avid horseback rider and had both a horse and a pony. Today's parasite was always one of the things I dreaded about summer. Gasterophilus intestinalis, the horse botfly, is an annoying creature to horses and horse owners alike. The females are large and aggressive and pester the horses until they eventually alight and deposit small eggs that stick to the horse's hair like glue, most often on their legs. The eggs itch and so the horse licks and bites at it, eventually swallowing the eggs. The larvae then live out the fall and winter months in the stomach of the horse, which can make the horse lethargic and induce colic. In the spring, they are passed out in the feces, where they pupate and then emerge as new - annoying - flies. We would always try to prevent infection by scraping the eggs of the horses' legs using specially made rough sponge-like things - I spent hours doing that each summer!
The photo shows a very heavy infestation of botfly larvae in the stomach of a horse and comes from this site.