Interestingly, this question popped up in keyword searches a number of times, so I’m going to briefly address it.
“Are myths fiction or nonfiction?”
The answer probably depends on who you ask and why. I imagine that if you ask an atheist, you’ll get the answer “fiction”. But in the wonderful world of the Dewey Decimal System, books (and other media) on mythology are in the 200s, the category for philosophy and religion. So for straight mythology or books about mythology, it’s considered nonfiction. Poetry (like Homer’s Odyssey will generally end up in the 800s, with other books of poetry. Yes, poetry is considered nonfiction.
Novels and stories inspired by mythology usually end up getting pulled from the 800s and end up shelved with fiction, though. So if you’re asking because you want to know where Rick Riordan’s books fall on the shelf, you’ll find those in fiction. And if you are asking about a graphic novel, it kind of depends on the library. Some libraries will shelve all graphic novels under 741.5, the number for that format, and some pull the graphic novels into a separate section and shelve them by either subject (my daughter’s elementary) or author (my son’s middle school).
So the answer is that, especially in the library, it’s complicated. And sometimes it is kind of hard to figure out. If you’ve encountered Percy Jackson’s Greek Gods”, it probably falls in nonfiction, even though it is written in the annoying contemporary voice of a fictional character(that’s just my personal opinion, my kids love it) and “updated” versions of many myths. But the novels will end up shelved in fiction. Ultimately, though, the myths of a culture are stories of their gods, and their religion, and as long as people believe in gods, mythology is nonfiction.
It occurs to me that, given that this site focuses on horror fiction, someone reading this might think “Well, what about the Cthulu mythos? That’s a mythology, right? Why isn’t Lovecraft in the 200s?” As it was originally the invention of one person recognized as a writer of fiction, and how that person felt about religion is publicly known, I don’t see why it would be anywhere except in fiction. If you do know a person who worships the Elder Gods, please encourage them to seek help.