A simple square tune in two equal parts. Key of A and fiddle might as well cross-tune (AEAE). Buckin Dun is also called Bobtailed Mule, but so are other old time tunes.
The original recording. Bobtailed Mule fiddled by KC Kartchner [slippery hill].
Hokum Hi-Flyers on stage [utoob] with harmonica.
Uncle Earl live from their pre-recording days [Internet Archive, track 2, music starts 1 minute in]
It is fun to say Buckin Dun at a jam.
A bucking dun is a grey-brown mule who bucks.
Bucking Mule is a completely different two-part old time fiddle tune in key of A.