In the very distant past, before the gods came to the world: dragons, fiends, elementals, aberrations, Fey, and nymphs were born from the “Big Bang” - the residual divine energy from the behemoths becoming the world. These lifeforms were already fairly sophisticated by the time Lagahashra and Adar began to experiment with creation. The taboo against worshipping beings other than gods stems in part from the gods' distrust of creatures that were of the behemoths, but not intentional creations of the behemoths.