The Collaborative International Dictionary of English v.0.48:

Hippogriff \Hip"po*griff\, n. [F. hippogriffe; cf. It.
   ippogrifo. See Hippopotamus, Griffon.] (Myth.)
   A fabulous winged animal, half horse and half griffin.
   --Milton.
   [1913 Webster]

The Devil's Dictionary (1881-1906):

HIPPOGRIFF, n.  An animal (now extinct) which was half horse and half
griffin.  The griffin was itself a compound creature, half lion and
half eagle.  The hippogriff was actually, therefore, a one-quarter
eagle, which is two dollars and fifty cents in gold.  The study of
zoology is full of surprises.