Most of the jackalopes you see mounted on the walls of restaurants and souvenir shops are fakes of course, but some people think that the legend of the jackalope began when people first noticed rabbits with a disease called Shope papillomavirus, which causes growths that frequently look like antlers. The illustration above was drawn by American naturalist Ernest Thompson Seton in 1937 and published in a review by Kreider (1981).
The rabbit virus has proven important in human cancer research, as Dan Japuntich writes:
John Kreider, MD and other researchers at Pennsylvania State University
College of Medicine have been using rabbits and this virus in research
on cancer. The objective of their research program is two-fold: to
determine the contribution of human papillomaviruses (HPV) to the
development of uterine cervical cancer and to develop effective means
for the prevention and treatment of infections. Kreider just recently
retired, but he and his researchers eventually did develop a vaccine
for HPV's. Other research into Shope papillomavirus is extensive,
including DNA analyses of its forms. William Phelps (1985) did
extensive work on the Shope virus by trapping wild cottontails in
Minnesota and identifying two major viral-specific RNA species.
Humans have also been known to "grow horns" in a similar way, as in the case of Mrs. Mary Davis (1558), pictured below.
I found these illustrations and much of my information at Chuck Holliday's excellent website. As he points out, you don't want to look at actual pictures of either rabbits or humans with these deformities if you are in any way squeamish. If you insist, though, you can follow the links he provides.