The jackalope in the picture above resembles a horned hare drawn by E. T. Seton and included in the 1909 edition of his Life Histories of Northern Animals on p. 671 of Volume 1. Seton says of this specimen (shown on the right), "Rabbits with horns are frequently found in the dry region of the West. These horns are a disease, said to be a sort of growth caused by the irritant power of a skin mite. I had seen several Cottontails so afflicted, and had heard of Black-tailed Jacks [jackrabbits] that were "horny," so that I was not surprised to learn that L.G. Fisher, of Chicago, had a horned Prarie-hare in his collection. He courteously permitted me to make the accompanying sketch (Fig. 181), but the specimen was in a sealed case, so that I could not complete the examination. The growth seemed to be of real horn and had no resemblance to the growths I have hitherto seen on Rabbits."
It seems most likely that Seton was shown a jackalope made with the horns of a goat mounted on the head of a jackrabbit, just like the specimen immediately above. However, he was correct about rabbits and hares in the American West having growths that resembled horns. This is the earliest reference I have found which states the possible real-life basis for the jackalope legend - horn-like tumors on rabbits' heads. For more on this, see the bottom half of my main jackalope page.




Lots more jackalope pictures are available here, a jackalope site posted by Mike Novak.