Why are they called hogs?

Not because of Harley Owners Group. And not because Harley tried (and failed) to copyright the term. It was because of these guys,
Harley Davidson's original race team of the early 1910's and 20's. After winning these guys would take their team mascot for a victory lap.

I lifted these photos from The selvedge yard where JP lays anything worth knowing at your finger tips. There is a ton of great stuff there, go check it out.