I recently bought Getz's Early Stan and was amused to read in the liner notes that for contractual reasons one of the album's sessions was originally issued in the name of "Sven Coolsen", Stan having considerable Scandinavian connections at that time. The pseudonyms adopted by jazz musicians have always intrigued me. I suppose the first I came across was Charlie Parker appearing under the name of "Charlie Chan" on the 1953 Miles Davis "Serpent's Tooth" session. My favourite, though, is Johnny Smith's elevation of his common-or-garden name to "Sir Jonathan Gasser" on the Jazz Studio 1 session.
Let me know of the ones you like. What's the earliest example of this practice and does it still go on nowadays? Of course, there may have been a thread already on this, but if there has, I couldn't trace it. If so apologies and please provide a link!