SUMPTER is a 7 letter word that starts with S.

1. \Sump"ter\, n. [OF. sommetier the driver of a pack
   horse; akin to OF. & F. sommier a pack horse, L. sagmarius,
   fr. sagma a pack saddle, in LL., a load, Gr. ? a pack saddle,
   fr. ? to pack, load; cf. Skr. saj, sa[~n]j, to hang on. Cf.
   {Seam} a weight, {Summer} a beam.]
   1. The driver of a pack horse. [Obs.] --Skeat.
   
   2. A pack; a burden. [Obs.] --Beau. & Fl.
   
   3. An animal, especially a horse, that carries packs or
      burdens; a baggage horse. --Holinshed.
   
   

2. \Sump"ter\, a.
   Carrying pack or burdens on the back; as, a sumpter horse; a
   sumpter mule. --Bacon.