Whether they exist or not, like Bigfoot, Yeti etc, they appear in folklore. New species are regularly found in the Vietnam, Laos, Thailand area. So it is likely that some soldiers may have talked about the "Rock Apes", "Batutut" or whatever just as folk discuss Yeti in Himalayas and Bigfoot in the Rockies. Add in the "old hand" - "New Guy" wind ups, a bit of marijuana and a whole mythology can be created and gain a life of its own.