The best information I can provide can be found on this web site:
http://ww2.lafayette.edu/~hollidac/jackalope.html
Don't be thrown by the fact that it is a "Jackalope" web site... just scroll down the page to the article titled "JACKALOPES: HORN-LIKE GROWTHS ON RABBITS AND HARES IS A REAL DISEASE." It's about the Shope papillomavirus in rabbits.
Unfortunately, no "cure" is mentioned, since they're still pursuing the actual causes of this disease.
Several years ago, I had a doe develop one of these on her spine and measuring about 2 inches in height. I proceeded to carefully remove it, whereupon it just "fell off" leaving a patch of smooth skin and no blood. It never returned, so I never thought any more of it.
I hope this helps.
Pat Lamar
President
Professional Rabbit Meat Association
http://www.prma.org/