Due to the combination of experimentation on engineering larger animals, the large amount of radioactive fallout causing rapid mutagenesis, and the high oxygen content on Calydon, large, feral variants of natural animals, known as kaiju have arisen and reside primarily in the Vast Deferens. These include Lizards, Rats, Scorpions, Snakes and others. Some of these creatures have regained names from terrestrial mythology, such as the great-winged Rocs.