where came the dragons from? Or the idea of dragons?

most likely the komodo dragon, and other large reptiles that kill people (with a little help from the human need to exaggerate), also the massive skeletons of beached whales and such probably scared early humans into believing the exaggerations.

"dragon" is a later variation/version of "drakon" which was pretty much a massive snake that could breathe fire, may or may not be poisonous, and could cause paralysis if u looked at its eyes, though some have wings, drakon's are very physically similar (sometimes) to basilisks but very different behaviours and details. basilisks vary in appearance in myths but commonly a large snake.

It's pretty likely that fossilized bones (especially dinosaurs) were the source of many dragon myths, as well as living giant reptiles like Nile crocodiles.

the Bible actually talks about giant beasts who "...eats grass like an ox. Behold, his strength in his loins, and his power in the muscles of his belly. He makes his tail stiff like a cedar; the sinews of his thighs are knit together. His bones are tubes of bronze, his limbs like bars of iron. “He is the first of the works of God; let him who made him bring near his sword! and later on it says of a species which shoot sparks from their mouths