The song was written by Piero Umiliani, and used in a Swedish soft porn movie before (you can easily find his version on filesharing networks). Lots of info on Piero
on this page. It may have also been Henri Salvado, who is on that page as well.
There are two versions of Mah Na Mah Na (or Mahna Mahna) by Henson's Muppets characters. Neither of them are Animal; according to the CD, it is sung by "Mahna Mahna & The Two Snowths". I don't know who voiced him, but I'd be willing to bet it was Jim Henson, since he also did Dr. Teeth and Rowlf. The famous one was 1977, on Sesame Street. The other one, based on the look of the characters, may have predated Sesame Street.
Oh, on further searching,
here's a page showing screenshots from different Muppets versions of Mah Na Mah Na.