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]