No foe of the Supermen aggravated them more than these pair of pest, those irksome imps from the 5th dimension of Zrfff who perpetually plagued the Men of Steel throughout the years. Using the vast powers provided them simply from being inhabitants of an advanced dimension, both Mxys often tricked the Supermen out of sheer jest more than malice. Yet each time, they would be tricked into saying their names backwards which drew them back to them home worlds for periods of approximately three months. Although they seemed to bend the rules a bit perhaps due to the nature of time being different in each dimension?
Only they each had different spellings to their names as well as different appearances. Whereas Zrfff-Two's Mxyzpltk was clad in a suit and derby hat, Zrfff-One's Mxyzptlk was dressed in more futuristic garments. The golden age imp was tricked by saying Klptzyxm while the silver age imp was maneuvered into saying Klptzyxm. However, while this was a mandatory exile for the latter-day loon, for his predecessor it was out of his own sense of honor, having been outsmarted by his adversary.
Eventually, Mxyzpltk was coaxed into merging with the Phantom Zone entity known as Aethyr in order to obtain unimaginable power. This new Mxyzpltk inflicted tremendous harm to Zrfff-One as well as destroying Bizarro World with its automaton inhabitants and sending the Kryptonite remains of Argo City plummeting to the streets of Metropolis. While the bad influence of Aethyr and his Kryptonian minion Thul-Kar caused Mxyzpltk to be uncharacteristically malicious, the non-influence of their counterparts the Chequerians who plagued Earth-Two's Wonder Woman meant that Mxyzpltk retired from his life as a prankster in the mid 1950s in peace.
And, in fact, there were others in their family who carried on their infamous legacy. In the 30th century of Earth-One, there was Mxyzptlk V who as the "Masked Man" terrorized that world and slayed each member of the Legion of Super-Heroes until its last surviving member, Superboy, tricked him into saying his name backwards. This resulted in his return to the 5th dimension, although where is uncertain since Brpxz was destroyed by his ancestor.
On Earth-Two, Mxyzptlk V's counterpart was Mixy Pixy who intervened in an adventure involving Hop Harrigan and Tank Tinker. He was far more benevolent, and like his cosmic twin after one appearance he retired... not having the stamina of his apparent brother Mxyzpltk. It may also be speculated that Mxyzptlk V rehabilitated years later and joined the descendant of Lex Luthor in aiding the now adult Legion of Super-Heroes on a case... whereby they would become members of the Legion.