For those looking for truely classic monsters heres a list of monsters appearing in the orignal Dungeons and Dragons Monsters and Treasures book:
Men
Goblin/Kobold
Orcs
Hobgoblins/Gnolls
Ogres
Trolls
Giants (including Hill, Stone, Frost, Fire, and Cloud)
Skeletons/Zombies
Ghouls
Wights
Wraiths
Mummies
Spectres
Vampires
Cockatrices
Basilisks
Medusae
Gorgons
Manticores
Hydras
Chimeras
Wyverns
Dragons (including White, Black, Green, Blue, Red and Golden)
Balrogs
Lycanthrophes (Werewolf, Wereboar, Weretiger, and Werebear)
Purple Worms
Sea Monsters
Minotaurs
Centaurs
Unicorns
Nixies
Pixies
Dryads
Gnomes
Dwarves
Elves
Ents
Pegasi
Hippogriffs
Rocs
Griffons
Invisible Stalkers
Elementals (Air, Earth, Fire, and Water)
Djinn
Efreet
Ochre Jelly
Black Pudding
Green Slime
Gray Ooze
Yellow Mold
Light Horse
Medium Horse
Heavy Horse
Draft Horse
Mule
Small insects or animals
Large insects or animals
Note that several iconic monters such as Liches, Beholders, Umber Hulks, and Beholders don't show up until the Greyhawk Supplement. The only Demon listed is the Balrog. One has to wait for Eltrich Wizardy for the others to appear.
P.S. I am still afraid of Black Puddings