Byth Rok and Erik Razar were motivated by their criminal tendencies which led to them gaining tremendous powers both diverse and deadly. As shape changers, they were formidable foes indeed.
Byth was from the same planet as Hawkman and Hawkgirl, known as Thanagar-One. When he broke into the laboratory of a prominent scientist, he stole a pill which he ingested. This gave him the ability to change his shape into that of any creature. After a failed duel with the Winged Warriors on Earth-One, he returned thanks to an ally in Jarl with another scheme which also led to his defeat.
Erik was from the same planet as that world's Hawkman and his ally the Flash, known as Earth-Two. When he was sentenced to be executed, the electricity had a metagenic effect on him allowing him to transform into any animal he desired to become. After a clash with the Scarlet Speedster, this defeat climaxed in his death. Prior to this, a scientist named Princess Helene had experimented with shape changing technology, only her battle with the Flash also led to her transformation into a black widow that quickly died.