From Muppet Wiki
(clockwise from bottom-left) Kermit, Robert Kraft, John Boylan and Don Henley.
Don Henley (b. 1947) is a member of The Eagles and also performs as a solo artist. He covered "Bein' Green" on Kermit Unpigged.
In 2002, Henley provided the singing voice for Tennessee O'Neil in the Creature Shop-effects movie The Country Bears.