The Harpers Ferry Horse Pistol was a percussion lock pistol developed in 1855. Harpers Ferry, being one of America's main weapon producers, decided to produce the Horse Pistol for use by cavalry units in the final years of large cavalry use (the late 1800's being the last major successful use of cavalry).
As name suggests, the Horse Pistol was used to take down enemy horses, and was usually issued to Cavalry. A long barrel was used so that this firearm would be more comfortably carried in a saddle holster. The Horse Pistol was also used as a duelling pistol as its predecessor (the Harper's Ferry Model 1805) had been.
To increase accuracy, if the situation allowed, the Horse Pistol could have a stock attached, mounted to the underside of the grip. Another useful feature was the fact that the ramrod was mounted on a swivel. That allowed safer reloading on the horseback, eliminating the risk to drop the ramrod.
The Horse Pistol had octagonal rifling, tapering out to circle at the muzzle. This was an increasingly common (but still relatively rare) feature at the time, with only the most modern (at the time) rifles being built with rifling. The addition of rifling meant that the Horse Pistol was generally more accurate (with a greater effective range) than either pistols or the majority of the muskets still in use at the time (e.g. the Springfield Model 1842).
The Horse Pistol fired the same .58in Minie Ball as the Model 1855 Musket had done. The Minie Ball was a relatively new form of ammunition at the time, designed to deform when the pistol was fired to engage with the rifling in the barrel.
The Harper`s Ferry Horse Pistol served in American Civil War, mostly among Confederate sharpshooters and officers mounted on horseback. A large element of the Horse Pistol's use, however, was fighting against Mexicans and Indians.
The Horse Pistol was favoured to the early Colt revolvers beginning to appear because of the significantly higher power/larger .58in diameter Minie Ball. Yet, with the advent of reliable and powerful revolvers, the Horse Pistol was forced out into history with the rest of muzzle-loaded firearms.