The Weight reader known simply as E-Rock recently commented about "Ya Mar," a long-running Phish cover tune presumably by The Mustangs. According to Phish.net FAQ:
This is an old Mustangs song, originally written by Cyril Ferguson. Mike went on a Carribean vacation and saw the Mustangs perform in Freeport, then bought the album (tape?) and brought it home. "Ya Mar" (sometimes written "Yamar" by fans) was on the tape (purportedly the only tune on it worth covering), though the Mustangs are guessed to have covered it from someone else.
This tune can also be heard on Mike Gordon's collaborative release with Leo Kottke, Sixty Six Steps. Notably, the album contains a cover of the Pete Seeger tune "Living in the Country." It all comes full circle.
While I couldn't locate the original either, here is a decent clip of Phish performing (most of) the song on 5/23/00 at Roseland Ballroom. The 1st set of that evening included: AC/DC Bag, Wilson, First Tube, Ya Mar > Mike's Song > Simple > It's Ice, When The Circus Comes, Back on the Train, Gotta Jibboo, Taste, Sleeping Monkey