Wikipedia classifies Jack under "Hoax/Mass Hysteria/Demon/Phantom". He sprang up (pun intended) in 1837 when people began to have "sightings" of him all around England, and it quickly snowballed into folklore, sensational fiction, and real panic. Basically, he was a man (or some humanoid variation thereof) who looked demonic, with claws on his fingers and glowing red eyes, and could breathe fire. His most notable feature was that he could jump about fifty feet in the air.
"One report claimed that, beneath a black cloak, he wore a helmet and a tight-fitting white garment like an oilskin". So he was basically like every super-villain ever, right down to the leotard.
I prefer the versions of him where he was reported as being dressed like a gentleman. I like to think of him going, "Oooh, tally ho!" as he jumps and breathes fire on people, like so:
So he had these "super powers", and used them for really unsuper purposes. If I was half jumping spider, half dragon, I would lay waste to London until my demands were met. I don't even have any demands, people, but I guarantee I'd think of some just to put my powers to good use. What did Jack do? He spooked some horses, molested some women, and generally acted like all Three Stooges wrapped up in one horrifying body.
Once, outside of a fair, "he tore the blouse off [a woman] . . . and scratched her stomach before bounding into the darkness." Once a woman was walking home and he "jumped out of the shadows and spat blue flames in her face, temporarily blinding her, then retreated into the darkness". What purpose did this serve? This is so stupid. Then he "jumped in the way of a passing carriage, causing the coachman to lose control, crash, and severely injure himself" before springing away into the night with maniacal laughter. When some of these attacks were publicized, tons of people wrote in saying they had similar experiences, but were too sheepish to go forward on their own. Vigilante groups formed and "chased him" all over, but he was always "too fast" and escaped. Uh huh.
He got blamed for all sorts of crimes or experiences, including, at its most severe, the murder of a 13-year old prostitute. But mostly his reported escapades were really harmless, stupid things. There were tons of army sententials who claimed that he kept jumping up into their sentry boxes, slapping them in the face, and then leaving. It's always the same--minor mischief for absolutely no reason. The legend emigrated to America where, in the 20th century, variations on his story were seen in Kentucky, Texas and Massachusetts. He was last reported seen in 2012. Come on, guys, REALLY?
So what are our theories?
1.) Many researchers think it's mass hysteria, or even willful lying by some people in order to feel special and included by the rest of the panicking populace.
2.) It is also thought that it could have roots in reality--that the Marquess of Waterford, as part of a bet or as part of his naturally deviant behavior, decided to dress up in costume and attack people. This then encouraged imitators to continue to dress up, like so:
3.) Then, of course, you get the people thinking he was a demon accidentally or purposefully brought to earth by Satan worshipers.
4.) Predictably, some current people think he was an extraterrestrial from a high-gravity world (OMG, it's John Carter in reverse. I subscribe to this theory now).
Anyway, that's the legend, and it's really daft. I wonder if they've searched for him on MonsterQuest.