The Collaborative International Dictionary of English v.0.48:

Megathere \Meg"a*there\, Megatherium \Meg`a*the"ri*um\, n. [NL.
   megatherium, fr. Gr. me`gas great + qhri`on beast.] (Paleon.)
   An extinct gigantic quaternary mammal, allied to the
   ant-eaters and sloths. Its remains are found in South
   America.
   [1913 Webster]

WordNet (r) 3.0 (2006):

megathere
    n 1: gigantic extinct terrestrial sloth-like mammal of the
         Pliocene and Pleistocene in America [syn: ground sloth,
         megathere]