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]

Moby Thesaurus II by Grady Ward, 1.0:

57 Moby Thesaurus words for "sumpter":
   Siberian husky, ass, beast of burden, bidet, camel, carriage horse,
   cart horse, cavalry horse, draft animal, draft horse, dray horse,
   driving horse, dromedary, elephant, fill horse, filler, gigster,
   hack, hackney, hinny, horse, hunter, husky, jennet, jument, lead,
   leader, llama, malamute, mount, mule, ox, pack horse, palfrey,
   plow horse, pole horse, polo pony, post-horse, reindeer, remount,
   rider, riding horse, road horse, roadster, rouncy, saddle horse,
   saddler, shaft horse, sledge dog, stalking-horse, sumpter horse,
   sumpter mule, thill horse, thiller, wheeler, wheelhorse,
   workhorse



U.S. Gazetteer Places (2000):

Sumpter, OR -- U.S. city in Oregon
   Population (2000):    171
   Housing Units (2000): 215
   Land area (2000):     2.180118 sq. miles (5.646480 sq. km)
   Water area (2000):    0.000000 sq. miles (0.000000 sq. km)
   Total area (2000):    2.180118 sq. miles (5.646480 sq. km)
   FIPS code:            71000
   Located within:       Oregon (OR), FIPS 41
   Location:             44.746229 N, 118.199149 W
   ZIP Codes (1990):     97877
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Sumpter, OR
    Sumpter