Well, it'd be hard for me to give one general answer to this that would accurately represent all religions, tales, and beliefs of ancient times. Especially considering the variety and diversity of the pagan religions.
The pagan Greeks, Egyptians, and Romans of antiquity witnessed natural phenomena and because of the lack of significant scientific explanation, concluded that things such as wind were really people in the sky blowing down upon the earth.
Their representations of these people in the sky were not intended to be "metaphorical" nor "symbolic", they were intended, albeit erroneously, to be taken literally.
The pagan Aztecs viewed the Sun and thought it was a sentient entity. This belief is represented in ancient Aztec writing and iconography. Did they intend their works to be considered "symbolic"?
Not at all.
They believed the Sun was literally a being worthy of worship.
It is true that they did attribute certain human emotions to powerful beings, but that is different from intending for it to be symbolic or metaphorical.
They believed these entities existed.
The educated pagans did begin to move away from the idea of weather being caused by divine beings, especially around the time of the great Greek philosophers.
These beliefs are now regarded as myth because modern technology and science enable us to know for sure that there are no physical people standing in the sky blowing, crying, or throwing electricity down upon the earth.
Dragons and sea-monsters have been and meant different things throughout history.
In some cases they were just tales and folklore, passed down through oral or written word. In other cases comets have been thought to have been snakes or dragons do to their "tails".
The origins of these ideas are difficult to know for sure.
Did these people really think they saw a sea monster? Did they see a shark or whale and thought of it as a "sea-monster"? Was this just an entirely made up story intended as myth?
We can't know for sure.
Well, no.
There is a difference between something that is intended to be metaphorical and something that was intended to be literal, yet was an erroneous observation. Such as the Sun being a sentient entity. That was intended to be literal, but was clearly an error.
I'm sure there are religious texts out there that were intended to taken entirely metaphorical, but the absence of verifiable historical or scientific fact doesn't necessarily mean it is intended to be a metaphor.
On top of all this, a text doesn't necessarily have to be one either end of the spectrum. There are texts about (or allegedly about) historical events that may have metaphorical grammar in them, without the entire text or story being metaphorical.
As far as the Genesis creation account goes. That's still open to debate by scientists, theologians, and historians.
I personally regard it as a metaphorical or creative account of the creation of the earth. Though that doesn't mean I regard the entire Book of Genesis as a pile of metaphors.
Basically what I'm getting at is that there are no general statements that can be used to accurately represent all religious texts or even all religious texts that lack clear historical or scientific facts.
Each text has to be examined on it's own. Grammar, historical context, etc. need to be taken into consideration.
Sorry for the long rant.