lusus naturae

from 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}]

from 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]