It’s been known for decades that animals such as chimpanzees seek out medicinal herbs to treat their diseases. But in recent years, the list of animal pharmacists has grown much longer, and it now appears that the practice of animal self-medication is a lot more widespread than previously thought, according to ecologists. Source: U of Michigan, Apr 11, 2013
Zoopharmacognosy is a term used to describe the process by which non-human animals self-medicate. In the domestic equine world this is a very difficult thing for the horse to accomplish as his world is typically very manipulated. However horses in a natural environment are perfectly capable of doing this. Daniel Janzen first observed this behavior in various wild animal species in 1978. It is an important behavior function for parasite control and general health – the domestic horse can be kept in such an environment!
Applied zoopharmacognosy can be done on a case-by-case basis, using plant extracts and/or essential oils. Carolyn Ingraham in the UK has been instrumental in bringing a scientific approach to this discipline.
The book, Wild Health: How Animals Keep Themselves Well and What We Can Learn From Them, by Cindy Engel is a good read on this subject.