D&D's bestiary doesn't come straight from the pen of Gygax, though. Most traditional D&D monsters are derivatives of the many fey beasties that inhabit folklore.
Gary didn't write all the monsters. The Tome of Horrors had some good notes on that in their updates, and the Fiend Folio was mostly TSR UK written (go flumph!).
But Gary did his own takes on lots of classic monsters, and to me his rules for how a vampire or werewolf or whatever work have become mine over the years -- not just in playing AD&D, but in my gut understanding of the myth.
I had fun talking with him about trolls here. I still have "bridge trolls" (lives under a bridge, turns to stone if exposed to light) as a homemade monster in my games. Gygax knew all about those myths too, and had insight into where they came from. Those stories were told to me as a kid, but I'm an American with mostly Irish cultural background, and some Norwegian, and my parents were both English majors, so I'm never entirely sure where my parents got their mythology from. I'm guessing it's a Scandinavian myth, since we have a stone troll statue under a bridge here in Seattle (which is heavily Norwegian -- watch some "Deadliest Catch" and check the last names if that's in any doubt!). Pretty much any tale of monsters, Gygax knew about it.
