This is the most common South Jersey explanation of what the Jersey Devil is; It is small. Not so small as to not be threatening, but shorter than the average teenage male. The creature is much faster-moving than a human. It has been compared to the Tasmanian Devil, (not the cartoon). (Maybe the cartoon too, I don’t really know.)
So what does it look like? There is your problem, the creature does not want to be seen. Apparently, the creature has a habit of leaving fast enough, that there is no real concrete description. After a few attempts at film documentaries, the Jersey Devil became down-right hostile towards anyone who specifically sought it out for a photo or video. The lucky ones simply had all their stuff broken or stolen in their sleep. The Bart Andrews story was much more tragic (Hint: Marie Clapper).
The Jersey Devil was never a friendly thing to run into. The creature creates mischief wherever it goes. Heaven help you though, if you have a camera.