I really, really recommend John Kirk's "Legendary Quest" source-books.
Freely available @ http://legendaryquest.netfirms.com/Download.htm
Especially if you're comfortable ignoring stats (or converting by the seat of your pants).
LQ is based entirely on actual European and Mediterranean folklore and
mythology. There's no fluff here.
The Tome of Terrors™ is an encyclopedia of monsters taken exclusively
from mythology and folklore. It also contains detailed descriptions
of all the various sub-races of the standard LQ races. For example,
did you know that, according to folklore, there are a number of elven
families, each having its own unique personality? You'll find the
light-hearted natures of the Tuatha De Dannan and Liosalfar elves
quite distinct from those of the dark and brooding Svartalfar or the
murderous Sluagh Sidh. Dreaming up gripping adventures for your
players proves far easier when you have a solid foundation to build
upon, and the Tome is as solid as it gets. The Tome of Terrors is
needed only by the game referee and, in fact, should be avoided by all
other players to ensure the greatest enjoyment of the game. (175
There's 2 more explicit "monster manuals" after that: Celtic Creatures and Nordic Nightmares, & Monsters of the Mediterranean (and a few books of magic, traps, etc).
John has done some serious folk-tale research and traced the linguistic and cultural roots of nearly every common fantasy monster. The editing seems a little idiosyncratic (his own ruleset/spell-list/etc), and sometime it's not clear when you're in a subtype/race or have moved on to the next creature... but the flavor and theme shine through. I've been planning on converting many to my Earthdawn campaign, as Earthdawn has a very bronze-age high magic Mediterranean/Russian-steppes feel.