Mxyztplk
The mysterious pixie from the 5th Dimension made his comic book debut in Superman 30 in September 1944. Story by Jerry Siegel, layouts by Joe Shuster, finished art by Ira Yarbrough. 
Per Richard Morrissey's correspondence with Jerry Siegel, the comic version was completed prior to the newspaper version by Ellsworth and Boring. .

By the way, it's pronounced

"Mix Yizt Pulk."

according to writer Alvin Schwartz.
Mxyztplk's 1944 comic strip debut by Whitney Ellsworth, Wayne Boring and Stan Kaye ran from February to July 1944.

In June 1953, the magical pixie got a new wardrobe, courtesy of Al Plastino. (Superman 82).  It was colored green.  When he next returned in Superman 86, January 1954, the costume had assumed it's orange with purple trim standard coloring. Thereafter it fluctuated back and forth for awhile before finally settling on orange.  This panel is from Superman 96 and shows the green version by Al Plastino.
This page is from Action 190 in March 1954 and shows Wayne Boring's interpretation of Mxy's orange new duds.
Mxy vanished for a couple of years in the mid-fifties.  By the time he returned, in Superman 131 in August 0f 1959, writer Jerry Coleman had forgotten how to spell it, and so he became "Mxyzptlk", instead.

Pronounced

"Mix Yez Spit Lick."

despite any other pronounciations that may have appeared from time to time.  Art here is by Al Plastino.
It remained only to retcon Mxyzptlk's antics back into Superboy's time to complete the gestation of the "Earth 1" version of the character. Here he is from Superboy 78, January 1960, "The Ghost of Jor-El", by Jerry Siegel and George Papp. 
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