Horseman

Horsemen were a guard archetype that possessed the ability to use a variety of weapons while on horseback.

Equipment
Horsemen were mounted on one of two varieties of horse, either the common horse or the warhorse.

They were equipped with both short-ranged weapons such as long swords or maces, and long-ranged weapons like throwing knives (for those mounted on the common horse) and crossbows (for those mounted on warhorses). They did not have long or heavy weapons, due to the fact that it was impossible to use two-handed weapons on a horse.

Combat
If a horseman detected Ezio Auditore while he was on foot, the horseman would either charge at him with his short-ranged weapon ready (if Ezio remained still) or would try to hit him with his long-ranged weapon (if Ezio fled or started Free Running).

Ezio could unsaddle a horseman by stabbing him off with a spear, or by slashing his charging horse, and causing it to trip. However, after this attack, both horse and rider would eventually get up from the floor, seeming completely unharmed. The horseman would then challenge Ezio on the ground, or attempt to re-mount.

Ezio could also kill still-mounted horsemen by shooting them down with the crossbow or pistol, or by throwing spears into them. He could also assassinate guards with his hidden blade while they were on their horses, and steal their mounts.

Trivia

 * Sometimes, a guard on foot (Particularly Leaders) will mount a nearby horse upon becoming hostile towards Ezio. If no free horse is available, he will hijack one from a passing civilian.
 * Seekers and Papal guards are the only types of guard that can climb on a horse, although the former are far more common.
 * Horsemen cannot be killed with the Poison blade while on horseback, and must be unsaddled first. If you use a poison dart however, there is a chance that the horseman will fall off.
 * Horsemen cannot be lured by thieves, attacked by mercenaries, or distracted by courtesans.
 * Completing the Courtesan Guild Challenges will enable courtesans to distract horsemen and make them dismount.
 * Certain mounted Hermeticists share the abilities of horsemen.
 * In the in-game help page, horsemen are called Leaders.

Source

 * Assassin's Creed: Brotherhood