Zoanthropy

from WordNet (r) 3.0 (2006)

zoanthropy
    n 1: the delusion that you have assumed the form of an animal

from The Collaborative International Dictionary of English v.0.48

Zoanthropy \Zo*an"thro*py\, n. [Gr. ? animal + ? man.] (Med.)
   A kind of monomania in which the patient believes himself
   transformed into one of the lower animals.
   [1913 Webster]