Horses appear to be quite susceptible to infection and diseases caused by epidemic influenza viruses. The first known outbreak was due to an unknown influenza virus type in horses in Canada and the USA in the 1870s that caused great disruption due to the dependence of society on horse power at the time.
The first well understood and isolated equine influenza virus (H7N7) emerged in the 1950s in Europe, and that virus spread among horses in many countries, including in the USA, but it died out in the late 1970s.
The current equine influenza virus is the H3N8 strain, which emerged in the early 1960s, being first seen in South America (Argentina and Uruguay) and then Florida in the USA. An independent spillover of a different avian H3N8 strain into horses took place in Jilin province in China, causing an outbreak from 1989-1990 before going extinct.