I once happened across one of these on the trail while hiking once:
They're called Hickory Horned Devil caterpillars, and for a moment the durned bugger startled the crap out of me. They're harmless, though, and turn into these:
The Regal Moth. Awesome looking creatures.
They both look kind of awesome
last year we had a plague of Buxsus moth caterpillars, while harmless to humans they devastated our hedges, they ate them back so far the hedge was dead this year, and we had to remove all the buxus hedges we had, it was heart breaking. The only way of getting rid of them is to spray with poison as they are an alien species to Belgium coming from China, our birds don't eat them, but with defoliating the hedges they have removed the habitat for other bugs and insects so the bird population has declined this year.