Also in the form megatherian, but I'm sure you all agree that megatherial is much more elegant in sound. The megatherium (coined from Greek meaning "large beast") was a New World Pleistocene megafauna, with an emphasis on the mega, being a ground sloth the size of a bull African elephant. If the 1911 Century Dictionary is to be believed, the figurative extension was coined as a nonce word in an 1896 letter to Nature. In fact, I'll quote the citation as a usage example, as the word also describes the style:
"Beautiful problems of the past of our island and the evolution of life were defaced by the disorderly offspring of a quite megatherial wit -- if one may coin such an antithesis to 'etherial'."