Chickens are a domestic animal. Jungle fowl are wild. They have different names to indicate they are different. Pure jungle-fowl absolutely exist, they are just rare. Many people on this forum own them. Go ask them about it. Get educated.
If junglefowl are chickens, then wolves are dogs, aurochs are oxen, etc... Again, there are excellent teachers here on BYC who can help people understand this better.
I am educated. I have a Ph.D. in earth science and have taught high school biology. Wolves are dogs (both are the genus Canis) and Aurochs are oxen (both are the genus Bos). Wolves and dogs both contain the same genetic information and both Aurochs and Oxen contain the same genetic information. The difference is not one of genetics but of selective breeding to bring to the front certain characteristics that lays within that genetic information. This is simple biology.