I think that the medieval writing on dragons was probably based on information from China where dragons were prominent in art. They no doubt had found many skeletons of monster animals which they called dragons, but which were in fact dinosaurs. There are large numbers of such skeletons in the Gobi desert. Where on earth were these dinos thriving 1000 years ago and where are their mini skeletons found????
As to skin being on mammoth bones that were more than 10,000 years old, that was certainly possible if they were in a desiccating sand dune environment. Mummies/bodies buried in Egyptian sands as much as 7,000 years ago still have skin.