Iron Age - you're talking the time of Tuatha De Danann in Ireland, Beowulf, Sigfried, and the Norse Sagas if you stick to northern Europe alone. So we're talking dragons, worms (more like Linnorn), giants, ogres, hags, trolls, fey, ghosts, ghouls, early vampires, lycanthropes, merfolk, selkies - really most of the traditional D&D monsters.
Not Bronze Age beasts like minotaur, sphinx, centaur, etc - Ancient Greek. Not the iconic D&D bugbear, illithid, carrion crawler (Gygax inventions). Not much on the demon, devil, angel which is really more modern (Christian Europe) or remnants of ancient gods. But most everything else.