I have never felt personally that the Bible spoke of an actual Dragon
Revelation 12
3And there appeared another wonder in heaven; and behold a great red dragon, having seven heads and ten horns, and seven crowns upon his heads.
4And his tail drew the third part of the stars of heaven, and did cast them to the earth: and the dragon stood before the woman which was ready to be delivered, for to devour her child as soon as it was born.
this is the last book of the new testament
the first book of the old testament has
Genesis 3
1 Now the serpent was more crafty than any of the wild animals the LORD God had made. He said to the woman, "Did God really say, 'You must not eat from any tree in the garden'?"
13 And the LORD God said unto the woman: 'What is this thou hast done?' And the woman said: 'The serpent beguiled me, and I did eat.' 14 And the LORD God said unto the serpent: 'Because thou hast done this, cursed art thou from among all cattle, and from among all beasts of the field; upon thy belly shalt thou go, and dust shalt thou eat all the days of thy life.
what form the serpent took up til that point is unknown although obviously from the biblical text it must have been an animal used as a food source "cursed art thou from among all cattle"
you have to go to earlier texts than the late in the day biblical one to find out
up til that point the creature known as the serpent had all the same characteristics i.e. evil, opposed to god,
but, and this is a big but, the serpent was way too large to fit into a tree