The Collaborative International Dictionary of English v.0.48:

Lusus naturae \Lu"sus na*tu"r[ae]\n. [L., fr. lusus sport +
   naturae, gen. of natura nature.]
   Sport or freak of nature; a deformed or unnatural production.
   [1913 Webster]

WordNet (r) 3.0 (2006):

lusus naturae
    n 1: a person or animal that is markedly unusual or deformed
         [syn: freak, monster, monstrosity, lusus naturae]