Below are a number of common horse diseases, syndromes and disorders that you might encounter as a horse owner. The links take you to horsehints.org articles that address these problems:

Cushing's Disease aka Cushing's Syndrome or Pituitary Pars Intermedia Dysfunction
EPM aka Equine protozoal myeloencephalitis
Equine Herpesvirus
Insect Bites and Stings in Horses
Lameness, Lameness in Gaited Horses, and Lameness Non Weight Bearing
Lyme Disease, One of our horses, Rusty, tested positive for Lyme Disease and he had the full treatment. Lyme Disease and Humans and Lyme Disease Report Cornell University
Lockjaw aka Tetanus
Skin Diseases: Fungus, infections, allergic reactions: Rain Rot, Rain Scald, Streptothricosis, Dermatophilus Congolensis, Ringworm and Horses, Horse Sweet Itch, Queensland Itch, Summer Itch or Culicoides Hypersensitivity, Tail Rubbing, Hives aka Urticaria in Horses, Equine Parasites, and External horse parasites: flies, nats, lice, mites, chiggers, mosquitoes, ticks, bots.
Strangles aka Shipping Fever
Equine Infectious Anemia or Swamp Fever

These are only a few of the problems you may encounter but this list will be helpful to you. Fopr a complete list of medical problems in horses please follow this link: Index Horse Medical