The four legged Dragons, in particular, seem to have the characteristics of fossil remains of a tricerotops I think.
if only it were that simple Beagle
but the actual evidence shows that the earliest great serpents were all sea creatures that had giant snake like bodies
these stories proliferate amongst coastal peoples.
the monsters with legs only ever appear in descendants of these cultures who over time and with the advent of agriculture found that they didn't need to live by the ocean any more to get enough food.
they started relying on cereal crops and farming instead
its as if the stories were so addictive that as the peoples became less familiar with the ocean they adapted the stories because the people wanted to keep telling them and wanted to have the monster in them still
in mesopotamia for instance the Babylonian serpent Tiamat is a sea serpent with a long snake like body
the assyrian version of Tiamat has as you described a much more quadraped like body
she also has wings
pictures from an early babylonian seal.(circa 1800bce) the extra detail shows several sumerian garbed figures and the general consensus is that this means the seal is one that has been carved and recarved over millenia keeping the original characters as they appeared on a sumerian seal.
Tiamat from the wall of Babylon look she suddenly grew legs (circa 1000bce)
assyrian version of Bel Marduk and Tiamat here she has sprouted wings as well (circa 500bce)
its like cinderella when it was originally written the heroine had a fur slipper
when the story was retold in france the slipper becomes glass
but in both cases she still has a slipper
the Noah story is a case in point
in its first telling the ark is made out of the wall of a reed hut
in the biblical version its made of wood and is huge
each time a story is retold it gets extra details
this is especially true when the story crosses a cultural divide