The Collaborative International Dictionary of English v.0.48:

Althorn \Alt"horn`\, n. [Alt + horn.] (Mus.)
   An instrument of the saxhorn family, used exclusively in
   military music, often replacing the French horn. --Grove.
   [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "althorn":
   alpenhorn, alphorn, alto horn, ballad horn, baritone, bass horn,
   brass choir, brass wind, brass-wind instrument, brasses, bugle,
   bugle horn, clarion, cornet, cornet-a-pistons, corno di caccia,
   cornopean, double-bell euphonium, euphonium, helicon, horn,
   hunting horn, key trumpet, lituus, lur, mellophone, ophicleide,
   orchestral horn, pocket trumpet, post horn, sackbut, saxhorn,
   saxtuba, serpent, slide trombone, sliphorn, sousaphone, tenor tuba,
   tromba, trombone, trumpet, tuba, valve trombone, valve trumpet