In appearance, Yog resembled a glowing green mass that could shift-shape but is usually took a humanoid shape. Because of this, Yog had no exact size and mass.
During the Cretaceous Period, Yog came to Terra in a small meteorite and crash-landed onto Serigo Island. Once he had fully recovered, Yog set out to conquer the primitive world and to do so, he required a fighting force. He found three of the island's inhabitants: a rock turtle, a cuttlefish and a stone crab. Yog then entered their bodies and began to mutate their DNA, causing them to transform into the kaiju known Kamoebas, Ganimes and Gezora. However Yog had soon lost control over the three monsters and as a result was killed, leaving the Kaiju to follow their own paths.
Tenrou Island Arc
While Yog doesn't appear in the story arc, his is brought up in a confrontation between Ganimes and Kamoebas on Tenrou Island during the war between Fairy Tail and the Earth Defenders against Grimoire Heart and the Earth Conquerors .
Mutating DNA: The DNA within Yog's cells had a mutating affect on other organism and so by entering other organisms, Yog could transform those organisms into kaiju.