The Collaborative International Dictionary of English v.0.48:

Single-foot \Sin"gle-foot`\, n.
   An irregular gait of a horse; -- called also single-footed
   pace. See Single, v. i.
   [1913 Webster]

         Single-foot is an irregular pace, rather rare,
         distinguished by the posterior extremities moving in
         the order of a fast walk, and the anterior extremities
         in that of a slow trot.                  --Stillman
                                                  (The Horse in
                                                  Motion.)
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Single-foot \Sin"gle-foot`\, v. i.
   To proceed by means of the single-foot, as a horse or other
   quadruped. -- Sin"gle-foot`er, n.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

single-foot
    n 1: a rapid gait of a horse in which each foot strikes the
         ground separately [syn: rack, single-foot]
    v 1: go at a rack; "the horses single-footed" [syn: single-
         foot, rack]