I like using wide-angle lenses for environmental portraits of people and animals. If you get close, you can get the subject up close and personal, but the wide angle shows all the surrounding “story”.
You do have to be careful. The closer something is to the wide lens, the more it will distort or elongate. A horse already has a “horse face,” so who cares if you make it even more “horse facey”. With a bride at a wedding, however, if you use a wide angle lens and give her a “horse face,” you might not get paid.
Irish horses near Dooagh, Achill Island, County Mayo, Ireland
Canon 1DS. Mark III. 16mm lens. ISO 400. f/14@ 1/125 second.