At some point before they knew about the Cybermen, the Lizard Kings fired missiles at an object that descended through Mondas' mesophere after a cosmic cloud appeared. The missile attack missed, but the recoil damaged the Cyber-Scout craft and the Cyberman inside, which had come from a time after Mondas had separated from its sister world, Earth. The Lizard Kings examined the Cyberman, whose voice box was damaged in the crash, through vivisection, and salvaged the craft, sending an augmented ape-servant through a pod engineered from the craft to test the Chief Scientist's claims that the cloud was a means of escape from the rogue planet that threatened Earth. On the other side of the cloud, the Cybermen wounded the ape creature in an era after Earth was no longer Mondas' sister world and examined it with vivisection. They sent a Cyber-Scout craft with a Cyberman inside through the cloud in their era because the ape's race was suitable for cyber-conversion. This was the very same Cyberman that inspired the Lizard Kings to send their servant through the cosmic cloud in the past. (COMIC: The Prodigal Returns)
Behind the scenes
Although published several years after the Silurians of Earth's redesign from the Doctor Who television story The Hungry Earth onwards, the Lizard Kings in the comic story The Prodigal Returns more closely resemble the Lizard Kings seen in the original 1990s run of The Cybermen comic series in Doctor Who Magazine and the Earth Silurians seen in the 1970 television story Doctor Who and the Silurians and the 1984 television story Warriors of the Deep.