The Collaborative International Dictionary of English v.0.48:

Sumpter \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.
      [1913 Webster]

   2. A pack; a burden. [Obs.] --Beau. & Fl.
      [1913 Webster]

   3. An animal, especially a horse, that carries packs or
      burdens; a baggage horse. --Holinshed.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Sumpter \Sump"ter\, a.
   Carrying pack or burdens on the back; as, a sumpter horse; a
   sumpter mule. --Bacon.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sumpter
    n 1: an animal (such as a mule or burro or horse) used to carry
         loads [syn: pack animal, sumpter]