A horse is a domesticated hooved mammal used during pre-history as a source of transport and labour. Primarily used as a beast of burden and mount, it has also been a source of milk, meat and leather.
Farriers, saddlers, leather-workers (reins, stirrups etc.) and riders work with horses. They are useful for knights to ride (it enables them to out-run the peasants easier). The catch is that, since the horse can carry their weight, the knights tend to wear heavier armour, and then they (the horses) being vulnerable to good missile fire, they (the knights) tend to find out why heavy clunky armour doesn't go well with fighting on rough terrain, against lightly-armoured, and therefore more manoeuvrable, opponents.
Comes in three flavours: stallion, mare and gelding. Geldings are castrated male horses; the prodecure is considererd necessary to make sexually-mature stallions more docile and rideable. Young male horses are called colts, and young female horses are called fillies. Horses under one year of age are called foals, regardless of sex.
In terms of the medieval life, there were several types of horses categorized by the jobs they were trained to do:
- War horses carried mounted fighters into battle. War horses are typically large and powerful in order to carry a fully-armored and armed human combatant. War horses are also trained to be combatants in their own right, often taught to rear, kick and bite opponents. The modern day Lippizaner Stallions still perform these combat maneuvers dubbed aires above the ground." 
- Ambling horses (which, in England, became known as "hackneys" after the village and marshes east of London where a breeding line was established) were used for light transportation.
- Working horses (i.e. agricultural proto-tractors) used to pull wagons and ploughs.
- Carrying horses which were the medieval equivalent of the commercial van.
Horses in the SCA
Horses in Re-Enactment
Horses are used in re-enactment in a number of capacities. Most commonly they are used as cavalry units on the battlefield such as re-enactments of the Battle of Hastings, Battle of Bannockburn and various 17th century battlefields such as Marston Moor, Edgehill and Naseby. In this capacity they tend to fight other cavalry units as horses can become dangerous to foot troops. They are also used in recreations of jousts.
- Equestrian activities