Yes, they do exist. I found this yesterday along with 3 other MONSTER ginseng plants that were 4 prongs. These plants were knee high on me and I am 6' 2". I plan on visiting this site again in August!!
This is the 3rd 5-prong that I have ever found. I really think that they are just a freak of nature, because normally all the ones I have dug are not any older than the surounding 4-prongs and big 3-prongs.