Septon Barth's claim on the origin of Targaryen dragons, is the truth, when you look at the narrative.

[maesternewton]

In TWOIAF, we found out that Septon Barth wrote a book on how Valyria acquired it's dragons.

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In Septon Barth’s Dragons, Wyrms, and Wyverns , he speculated that the bloodmages of Valyria used wyvern stock to create dragons. Though the bloodmages were alleged to have experimented mightily with their unnatural arts, this claim is considered far-fetched by most maesters, among them Maester Vanyon’s Against the Unnatural contains certain proofs of dragons having existed in Westeros even in the earliest of days, before Valyria rose to be a power.

The World of Ice & Fire, Beyond the Free Cities: Sothoryos.

 

In his book, Barth makes the bold claim that Old Valyria made their dragons from bloodmagic. I do believe his claim to be the truth. Why?

 

Most of what Septon Barth claims, turns out to be the truth.
Septon Barth made another bold claim that the Children of the Forest taught the men of the Night's Watch how to speak through ravens, that using ravens as a means to transport letters is a degraded form.

This claim turns out to be the truth, despite the Maesters disagreeing with this notion.

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"Do all the birds have singers in them?" "All," Lord Brynden said. "It was the singers who taught the First Men to send messages by raven … but in those days, the birds would speak the words. The trees remember, but men forget, and so now they write the messages on parchment and tie them round the feet of birds who have never shared their skin." A Dance with Dragons, Bran III.

He also claims that the seasons are a result of the higher mysteries and magic rather than natural causes, a notion that is again scoffed at by the Citadel. A notion that our author GRRM has confirmed to be the truth.

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It’s not another planet. It’s Earth. But it’s not our Earth. If you wanted to do a science fiction approach, you could call it an alternate world, but that sounds too science fictional. Tolkien really pioneered that with Middle Earth. He put in some vague things about tying it to our past, but that doesn’t really hold up. I have people constantly writing me with science fiction theories about the seasons — “It’s a double star system with a black dwarf and that would explain–” It’s fantasy, man, it’s magic.

By looking at those two examples, it's reasonable then to establish Barth as a reliable source of information.

 

Blood magic seems to be capable of doing what Barth claims.
According to TWOIAF, blood magic is capable of breeding two distinct creatures to create an entirely different one.

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In the dungeons of Gogossos, torturers devised new torments. In the flesh pits, blood sorcery of the darkest sort was practiced, as beasts were mated to slave women to bring forth twisted half-human children.

Whatever Aerea encountered during her flight to Old Valyria seems to be a result of blood magic.

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“The things…Mother have mercy, I do not know how to speak of them… they were…worms with faces…snakes with hands…twisting, slimy, unspeakable things that seemed to writhe and pulse and squirm as they came bursting from her flesh. Some were no bigger than my little finger, but one at least was as long as my arm…oh, Warrior protect me, the sounds they made…

Fire & Blood, Jaehaerys & Alysanne, Their Triumphs and Tragedies.

 

Dragons do resemble wyverns and firewyrms
After the Battle of Rook's Rest, Sunfyre is too injured to take flight and instead remained at the Rook's Rest. During this time, it's likened to wyrm by those that witness it crawling throughout the place.

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The king’s dragon, Sunfyre, too huge and heavy to be moved, and unable to fly with his injured wing, remained in the fields beyond Rook’s Rest, crawling through the ashes like some great golden wyrm.

Fire & Blood, the Dying of the Dragons, the Red Dragon and the Gold.

 

When Daenerys takes Quentyn to see her chained dragons, she wonders whether her dragons can dig tunnels similar to firewyrms. They could, to a certain degree.

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He's made himself a cave, the prince realized. A burrow in the brick. The foundations of the Great Pyramid of Meereen were massive and thick to support the weight of the huge structure overhead; even the interior walls were three times thicker than any castle's curtain walls. But Viserion had dug himself a hole in them with flame and claw, a hole big enough to sleep in.

A Dance with Dragons - The Dragontamer

 

The strongest evidence of all, we find in Fire & Blood. Laena Targaryen, the daughter of Baela and Alyn, has an egg that hatches a creature that is for all intents and purposes, a wyrm.

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“Her parents' pride and pleasure quickly turned to ash, however; the dragon that wriggled from the egg was a monstrosity, a wingless wyrm, maggot-white and blind. Within moments of hatching, the creature turned upon the babe in her cradle and tore a bloody chunk from her arm. As Laena shrieked, Lord Oakenfist ripped the "dragon" off her, flung it to the floor, and hacked it to pieces.” Fire & Blood

When you look at all these points, the most logical conclusion one can come to, is that yes. Targaryen dragons, or dragons from Old Valyria are a result of sorcerery and magic done by the Valyrians.

[Craving Peaches]

Aside from your points there is also nothing which refutes Barth's points (I think),* and in AGoT it is mentioned that the dragons were 'bred for war', which lines up with the idea of a magic-influenced crossbreed of wyverns and fireworms.

*Apart from potentially the second moon myth implying the dragons are 'natural' magic creatures, which I think is a very weak counter, and the character who makes this claim subsequently dies of essential fatty acid deficiency, proving how weak their claim was.

Edited June 9, 2023 by Craving Peaches

[Aebram]

Barth was a very wise man, but not infallible. There are hints in the books that dragons existed long before the Valyrian empire. And the George himself has said that "there were dragons all over, once."

https://www.westeros.org/Citadel/SSM/Entry/Dragons_in_Westeros/

It may be that the Valyrians were the first to ride dragons. I don't think there's any evidence of dragonriders before the rise of Valyria. Perhaps they used their genetic magic to create dragons that were able to bond with and be controlled by men.

[Craving Peaches]

29 minutes ago, Aebram said:

There are hints in the books that dragons existed long before the Valyrian empire. And the George himself has said that "there were dragons all over, once."

They can easily just be a different kind of dragon though.

[maesternewton]

On 6/10/2023 at 10:07 AM, Aebram said:

Barth was a very wise man, but not infallible. There are hints in the books that dragons existed long before the Valyrian empire. And the George himself has said that "there were dragons all over, once."

https://www.westeros.org/Citadel/SSM/Entry/Dragons_in_Westeros/

It may be that the Valyrians were the first to ride dragons. I don't think there's any evidence of dragonriders before the rise of Valyria. Perhaps they used their genetic magic to create dragons that were able to bond with and be controlled by men.

True, they are hints dragons existed before Old Valyria.

The most logical conclusion I have come to on that is that Targaryens reinvented the wheel, so to speak. Perhaps an empire, Great Empire of the Dawn, before that knew how to breed dragons similar to the Valyrians. They eventually learned how to do so by acquiring the knowledge on blood magic. 

[weidback]

14 hours ago, maesternewton said:

The most logical conclusion I have come to on that is that Targaryens reinvented the wheel, so to speak. Perhaps an empire, Great Empire of the Dawn, before that knew how to breed dragons similar to the Valyrians. They eventually learned how to do so by acquiring the knowledge on blood magic. 

I imagine that they bred and cross bred existing creatures to suit their purposes just as humans created cattle from aurochs

Perhaps there always were dragons in some form - but the original weren't suitable for war and conquest. Too small to ride, unable to bond with humans, and their shortcomings were overcome through cross breading and blood magic