the name "horse pistol" in the Civil War designates single shot percussion pistols, due to the use of revolvers obsolete at that time. Nevertheless during the war they were still in use, mostly the following models:

Army:
M 1836 flintlock converted to percussion (Makers: Waters, Johnson), .54 cal. smoothbore
M 1842 percussion (Aston, Aston & Co, Johnson, Palmetto Armory - the latter was made in the South before the war but is regarded as Confederate),.54 cal smoothbore.
In 1855 a pistol carbine (percussion pistol with detachable shoulder stock) was introduced for some cavalry regiments. With the exception of a handfull of prototype specimens at Harpers Ferry, all were produced in Springfield Armory, .58cal rifled for Minié. It has the Maynard tape primer. The Confederates came in posession of parts and produced a few at Fayetteville (without the complicate tape primer).

Navy:
M 1826 flintlock converted to percussion navy pistol, with naval belt hook,(North, Evans) .54 cal. smoothbore
M 1842 (a.k.a.1843) percussion navy pistol with "box lock" (Ames, Deringer), also used by US Revenue Service, .54cal smoothbore.

The Confederates used perhaps also the J&F Garrett pistol of my collection , a pistol made with use of parts of the M 1842 Army pistol, but origins of this model are not clear. The use of European single shot pistols is improbable, perhaps with the exception of the German Federal Navy pistol M 1848 (unproven).

Best regards,
Marc