origins of mythological critters?

I always wondered where writers of Greek mythology got their ideas for a lot of the creatures in the stories.

The only one I know for certain is cyclops -- the likelihood that they found the skulls of extinct dwarf elephants whose nasal cavities resembled a single eye socket inspired the cyclops.

But what about all the other creatures in mythology and ancient stories?  Did they come from similar misunderstandings, or something as simple as someone's drug-induced hallcination?

They were insane. Sanity is a fairly recent thing. Some suggest the first name man was Voltaire.

sane man*