The Nephilim (Hebrew נְפִילִים) are, are ... well, no one has any idea really. They are mentioned in the Bible, though. But apparently not in previous mythology. The Septuagint, which underlies most Christian Bibles, translates the word as "γίγαντες," "giants," which is the word used in both the King James and Douay-Rheims versions in English.
Depending on your point of view they are (a) a third category of creatures separate from Men and Angels, (b) Aliens, (c) Giants, (d) Human-Angel hybrids, or (e) the result of the Bible being an inconsistent mishmash of previous mythologies.
“”The Nephilim were on the earth in those days—and also afterward—when the sons of God went to the daughters of men and had children by them. They were the heroes of old, men of renown.
“”And they spread among the Israelites a bad report about the land they had explored. They said, "The land we explored devours those living in it. All the people we saw there are of great size. 33 We saw the Nephilim there (the descendants of Anak come from the Nephilim). We seemed like grasshoppers in our own eyes, and we looked the same to them."
 Hoax Spottings
Some internet pranksters have attempted to pass off manipulated images of giant skeletons at archaeological dig sites as real. These images have been claimed as proof of the giants mentioned in the Bible, the Qu'ran, or Hindu texts, depending on the story.