riderless

From rider +‎ -less.

riderless (not comparable)

1. Without or deprived of a rider.
   • 1846 October 1 – 1848 April 1, Charles Dickens, “Chapter 46”, in Dombey and Son, London: Bradbury and Evans, […], published 1848, →OCLC:

     […] the spectacle of Rob the Grinder returning down the street, leading the riderless horse, appeared to have some extraneous interest […]