The Collaborative International Dictionary of English v.0.48:

Wallaby \Wal"la*by\, n.; pl. Wallabies. [From a native name.]
   (Zool.)
   Any one of numerous species of kangaroos belonging to the
   genus Halmaturus, native of Australia and Tasmania,
   especially the smaller species, as the brush kangaroo
   (Halmaturus Bennettii) and the pademelon (Halmaturus
   thetidis). The wallabies chiefly inhabit the wooded district
   and bushy plains. [Written also wallabee, and whallabee.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

wallaby
    n 1: any of various small or medium-sized kangaroos; often
         brightly colored [syn: wallaby, brush kangaroo]