The Coehorn mortar, a small muzzle-loading mortar, caliber generally 5.82 inches, is named for it's Dutch inventor, Barron Menno van Coehorn (1641-1704). Mounted on a block or platform, it was portable, easily adjusted, took little powder, and was particularly effective in sieges. The US Army had a 24-pounder brass Coehorn that weighed 164 pounds, or 296 pounds when mounted on it's four-handled oak mortar bed. Two men could move this mortar, but four men could better maneuver and rush this mortar into position in unprepared locations. Explosive shells could be lobbed into masked targets from 50 to 1,200 yards with the Coehorn.
Two possible answers depending on exactly what was wanted. The coehorns as it relates to the Civil War were muzzle loading mortars. Generally, the Union used 24 lb coehorns and the Confederates used 12 pounders.
As it relates to the question of what kind of "horn" was a coehorn?, the answer would be a mortar that the Spanish called a "cow horn."
"The small mortars called coehorns were invented by the famed Dutch military engineer, Baron van Menno Coehoorn, and used by him in 1673 to the great discomfit of French garrisons. James Oglethorpe had many of them in his 1740 bombardment of St. Augustine when the Spanish, trying to translate coehorn into their own tongue, called them cuernos de vaca—'cow horns.'"
It was a “big” horn that made lots of noise when used against the enemy and unlike most “horns”, it was not made of brass, but bronze and it weighed in at almost 300 pounds, as it fired its’ 16+ pound shell over 1,000 yards - but more importantly - it would never be “music to the ears” of its’ intended audience.
“The Coehorn mortar, a small muzzle-loading mortar, caliber”. http://civilwarwiki.net/wiki/Civil_War_Era_Mortars