The Collaborative International Dictionary of English v.0.48:

Capybara \Ca`py*ba"ra\, n. [Sp. capibara, fr. the native name.]
   (Zool.)
   A large South American rodent (Hydroch[ae]rus capybara)
   Living on the margins of lakes and rivers. It is the largest
   extant rodent, being about three feet long, and half that in
   height. It somewhat resembles the Guinea pig, to which it is
   related; -- called also cabiai and water hog.
   [1913 Webster]

WordNet (r) 3.0 (2006):

capybara
    n 1: pig-sized tailless South American amphibious rodent with
         partly webbed feet; largest living rodent [syn: capybara,
         capibara, Hydrochoerus hydrochaeris]