In Genesis 6:2 it says, ". . . the sons of God saw that the daughters of men were beautiful; and they took wives for themselves, whomever they chose." In 6:4 we are told "the Nephilim [giants] were on the earth in those days. . . the sons of God came in to the daughters of men, and they bore children to them. These were the mighty men who were of old, men of renown." This is from the New American Standard translation, perhaps the most literal of tranlations. Some of the older ones have "giants" instead of Nephilim. The root of the word means "fallen". Whether this is in the sense of fallen angels, or demons, or men who "fall" upon women, is debatable. The term "sons of God" seems generally to mean angelic beings. There are, as far as we are told, two classes of created beings: humans and angels. Demons are angels who rebelled against God. Demons seem to be able to take physical form and interact with humans; there are a number of instances in the Bible. The resulting offspring might well be giants, "mighty men . . . of renown".
Aside from Biblical references to Goliath and other giants, there have been numerous discoveries of giant burials: great skeletons measuring seven to twelve feet in height. They were often buried with armor and weapons of great size and weight. I believe these were descendants of demon-human interbreeding: nephilim. They originally possessed great intelligence and strength. The pantheons of "gods" refer to these nephilim. In other words, Zeus and Thor existed as actual beings. Any preternatural powers they might have possessed would have come from their demonic sires. Great intelligence, demonic knowledge, and great physical abilities would have been enough to dominate their world. Several epistles of the New Testament, Peter and Jude, indicate that God chained some of the fallen angels responsible in deep pits (i.e., Hell).
The fabulous creatures of myth, legend, and folklore (MLF), such as pegasus and centaurs, were actual beings created by demonic manipulation of genetic material. This, I think, is perhaps the main reason why God chose to wipe out most life on Earth. The Enemy perverts, but only God can create. "There were giants in the earth in those days . . ." Until the Flood. . . .