“The jackalope is a tricksome critter! Dangerous to all men and beasts!” It was a tale often told around campfires in the Old West. It looked like a jack rabbit, but wore a magnificent rack of antlers on it’s head. It was a fierce fighter, an enviable ventriloquist, and a shameful lush. It’s milk came pre-homogenized and could be drunk as a powerful aphrodisiac. Too bad they were so rare, since they only bred during thunderstorms.
Of course, now we know that jackalopes don’t exist, but at the time who could say? Before the mid-1800’s, the American continent was mostly unexplored by Europeans and many simply didn’t know what to expect. Some of these ‘fearsome critters’ of legend were obvious gags (like the axehandle hound, which ate only axehandles), but remember that during the Lewis and Clark Expedition, Thomas Jefferson explicitly reminded them to keep an eye out for giant ground sloths.
Now, this doesn’t mean that you won’t ever see a horned rabbit. Both wild and domesticated rabbits are susceptible to a virus known as Shope papilloma, which can cause cancerous tumors to sprout on the rabbit’s head and face. Some of these can get quite large. They can even be fatal. A popular theory is that these infected rabbits might have started the whole jackalope craze. The idea is that, to a casual observer, these growths might have looked like tusks or antlers. People a couple hundred years ago would have had no idea it was a virus. This might explain why other horned rabbits appeared in other cultures as well (like the European wolpertinger or the 16th century Lepus cornutus).
Sources: Museum of Hoaxes