Tolkien's mythology as he wrote it being very Western had only Evil Dragons.
Contrary to how some may make it sound it's not solely because
of The Bible the West tends to view Dragons/Serpents as Evil (in fact
there is overlooked Biblical support for good Dragons). It comes also from
Ophion, Hydra and Ketos of Greek Mythology, Jormungander and Niohoggr of
Norse Mythology, and the Grendles and Dragons of Beowulf.
Mythology usually has Dragons as Benevolent, though Japan has some evil
ones too. Many are Dragon Kings who rule the seas.
Since we readers and writers of Fan Fiction are
always interested in expanding and diversifying Tolkien's world, (as
detailed as he was he did leave plenty to the imagination). I've
contemplated how hypothetically possible it could be to put Benevolent
Dragons in Arda.
The origins of the Dragons that served Morgoth
are not directly explained, a common theory is the originals were fallen
Maia who Morgoth then bred. That Glarung seems to be equal in rank to
both Sauron and Gothmog, (both Maia, one a leader of other Maia),
suggests to me he's probably also a Maia. Which has me thinking that if an
evil Maia could take that form, there is no reason a good Maia couldn't
take a similar form.
So perhaps there could be Benevolent Dragons
known in the far Eastern lands of Arda, to the East of anything seen on
LOTR's maps. Perhaps something like Valoo from The Legend of Zelda:
The Wind Waker. In one scene Valoo and the Rito pretty much serve the
same function as the Eagles.
Someone already wrote one Fan Fiction for that concept.
If you're curious about my Biblical
statement. For Starters Jesus used the Brazen Serpent that Moses raised
up in the wilderness as a symbol of himself in John 3. The Hebrew word Seraph is
used in that account where it is made a synonym for Nahash (Serpent in
Genesis 3), in fact the word "serpent" may come from Seraph ultimately. Isaiah described Seraphim circling the Throne of Yahuah,
leading many to think of the term as a synonym for Cherubim. Also in
Job, God speaks of Behemoth and Leviathan as creations he's very proud
of. All these words can also be linked to Satan (except Behemoth), but
Satan was one of 5 Cherubim, the other 4 didn't fall.
When comparing Ezekiel's descriptions of the Cherubim with John's in Revelation 4. It seems like each one is particularly associated with one of the 4 types of animals, yet at the same time they each have traits of each, Eagle's wings, cloven hoofs, human eyes and bodies like lions. If Satan is the 5th Cherub and the Serpent/Dragon one, that the other 4 could also have some serpentine association. And it was in this form that Isaiah saw them.