Fly Agaric, Amanita muscaria. Usually found in proximity to pixies, and reputed to be an ingrediant in witches' flying ointment.
It contains a pretty powerful hallucigenic that gives a sensation of flying. Sami shamans used to give it to reindeer, then drink the reindeer piss - making dosage calculations much easier. Another side effect is raised temperature, and it has been suggested that reindeer that have ingested the stuff, reduce body temp by pumping blood through surface blood vessels. Because of the insulating properties of their coat, the near-surface blood flow concentrates at the nose - hence Rudolph's red nose, and the idea of flying reindeer.