There is nothing mythic or legendary about a goliath. In mythology, Goliath was just a really big guy. A human guy.
Maybe. It depends on which of the equally orthodox and traditional interpretations of the word Nephilim that you subscribe to.
As for Goliath in the Bible, although orthodox hermeneutics would deduce that he is a gigantic man, more imaginative interpretations have occurred over the millennia, including the idea that he was descended from the wayward Nephilim, the "sons of God" who were "giants in the earth in those days."
I would dispute the labeling of one interpretation as being orthodox and the other as not. I would also dispute the titling of the Nephilim as the Sons of God. My defense is going to be fairly lengthy. You are warned.
In the bible, the book of Genesis, there is a story, where the Sons of God see the Daughters of Men and are attracted to them; their progeny are Nephilim, or Giants. These Nephilim are wicked bad crazy wicked, and God sees all their evil deeds and despairs of them and decides to kill them all. And then the flood. Yeah, that's right. God decided to single-handedly kill every last Nephilim on earth. Unfortunately, it didn't quite work, because later on the Hebrews enter into a Land of Giants after fleeing from Egypt, and even later David kills himself a giant named Goliath.
So, who are these Nephilim, exactly? There are two interpretations, both of them equally orthodox. The first, which is the view held by most contemporary churches, is that the Sons of God were the descendants of Seth and the Daughters of Men were the descendants of Cain. Their offspring, then, the Nephilim (Giants), just as Ovarwa says above, would have been entirely human.
The second interpretation is that the Sons of God were not human at all but were angels, also called the Watchers. And their offspring, the Nephilim (Giants), would not have been human, but rather a hybrid half-human half-angel.
There is no direct textual evidence for either of these interpretations within the bible itself. The bible makes it clear only that the Nephilim were the progeny of the Sons of God and the Daughters of Men, and that they were Giants, and that they were entirely evil and the reason for the flood. And that's all. Assuming giants are simply really big people is the easy interpretation, given the lack of evidence for any other.
However, there are a couple of apocryphal texts, namely the Book of Enoch and the Book of Jubilees, that are unambiguous in stating that the Sons of God were angels.
Yeah, but those books aren't in the bible. So why should anybody pay attention to them? For exactly one reason: The Book of Enoch is itself quoted in the bible. It is, IIRC, the only extra-biblical book to be so quoted. Which means that the Apostle Jude, for one, believed it and took it to be true.
1st, there is no cool lore about them.
Goliath definitely suffers from some drawbacks, mainly Identity Crisis.
You know, maybe there isn't any "cool D&D lore" about Goliaths, but a race of creatures that are half-angel and all evil, that God himself decided to declare war on, well man that's about as cool as it comes.