The Collaborative International Dictionary of English v.0.48:

Yapock \Ya"pock\ (?; 277), n. [Probably from the river Oyapok,
   between French Guiana and Brazil.] (Zool.)
   A South American aquatic opossum (Chironectes variegatus)
   found in Guiana and Brazil. Its hind feet are webbed, and its
   fore feet do not have an opposable thumb for climbing. Called
   also water opossum. [Written also yapack.]
   [1913 Webster]
   [1913 Webster]