The Collaborative International Dictionary of English v.0.48:

Burden \Bur"den\ (b[^u]"d'n), n. [Written also burthen.] [OE.
   burden, burthen, birthen, birden, AS. byr[eth]en; akin to
   Icel. byr[eth]i, Dan. byrde, Sw. b["o]rda, G. b["u]rde, OHG.
   burdi, Goth. ba['u]r[thorn]ei, fr. the root of E. bear, AS.
   beran, Goth. bairan. [root]92. See 1st Bear.]
   1. That which is borne or carried; a load.
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            Plants with goodly burden bowing.     --Shak.
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   2. That which is borne with labor or difficulty; that which
      is grievous, wearisome, or oppressive.
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            Deaf, giddy, helpless, left alone,
            To all my friends a burden grown.     --Swift.
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   3. The capacity of a vessel, or the weight of cargo that she
      will carry; as, a ship of a hundred tons burden.
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   4. (Mining) The tops or heads of stream-work which lie over
      the stream of tin.
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   5. (Metal.) The proportion of ore and flux to fuel, in the
      charge of a blast furnace. --Raymond.
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   6. A fixed quantity of certain commodities; as, a burden of
      gad steel, 120 pounds.
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   7. A birth. [Obs. & R.] --Shak.
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   Beast of burden, an animal employed in carrying burdens.

   Burden of proof [L. onus probandi] (Law), the duty of
      proving a particular position in a court of law, a failure
      in the performance of which duty calls for judgment
      against the party on whom the duty is imposed.
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   Syn: Burden, Load.

   Usage: A burden is, in the literal sense, a weight to be
          borne; a load is something laid upon us to be carried.
          Hence, when used figuratively, there is usually a
          difference between the two words. Our burdens may be
          of such a nature that we feel bound to bear them
          cheerfully or without complaint. They may arise from
          the nature of our situation; they may be allotments of
          Providence; they may be the consequences of our
          errors. What is upon us, as a load, we commonly carry
          with greater reluctance or sense of oppression. Men
          often find the charge of their own families to be a
          burden; but if to this be added a load of care for
          others, the pressure is usually serve and irksome.
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WordNet (r) 3.0 (2006):

beast of burden
    n 1: an animal such as a donkey or ox or elephant used for
         transporting loads or doing other heavy work [syn: beast
         of burden, jument]

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "beast of burden":
   Siberian husky, ass, camel, draft animal, dromedary, drudge,
   elephant, fag, galley slave, greasy grind, grind, grub, hack,
   horse, husky, llama, malamute, mule, ox, pack horse, plodder,
   reindeer, slave, sledge dog, slogger, sumpter, sumpter horse,
   sumpter mule, swot, workhorse