Source : Webster's Revised Unabridged Dictionary (1913)

Ahorseback \A*horse"back\, adv.
   On horseback.

         Two suspicious fellows ahorseback.       --Smollet.

Source : WordNet®

ahorseback
     adj : traveling on horseback; "a file of men ahorseback passed by"
           [syn: {ahorse(p)}, {ahorseback(p)}]
     adv : on the back of a horse; "he rode horseback to town";
           "managed to escape ahorse"; "policeman patrolled the
           streets ahorseback" [syn: {horseback}, {ahorse}]