Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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