The Three Wild Dragons (Spoilers Extended)

Then there were three wild dragons that might be tamed if riders could be found: the Cannibal, said by the smallfolk to have lurked on Dragonstone even before the Targaryens came (though Munkun and Barth are dubious of this claim); Grey Ghost, shy of people, gorging on fish it plucked from the sea; and the Sheepstealer, brown and plain, preferring to feed on what sheep it could steal from the sheepfolds. -TWOIAF, The Targaryen Kings: Aegon II

The Three Wild Dragons seem much different than their Valyrian Counterparts

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I think its possible that the Cannibal is one of these non Targaryen dragons and it later laid the eggs or is from the same lineage of eggs resulting in Grey Ghost and Sheepstealer.

The Cannibal which may or may not have been from before the Targaryens arrival is at least the "oldest and largest of the wild dragons".

the Cannibal, said by the smallfolk to have lurked on Dragonstone even before the Targaryens came (though Munkun and Barth are dubious of this claim)

and:

The largest and oldest of the wild dragons was the Cannibal, so named because he had been known to feed on the carcasses of dead dragons, and descend upon the hatcheries of Dragonstone to gorge himself on newborn hatchlings and eggs. Would-be dragontamers had made attempts to ride him a dozen times; his lair was littered with their bones.

None of the dragonseeds were fool enough to disturb the Cannibal (any who were did not return to tell their tales).

Unlike Vermithor/Sea Smoke/Silverwing who allow known dragonseeds to mount them, the Cannibal/Grey Ghost don't ever get bonded.

Some sought the Grey Ghost, but could not find him, for he was ever an elusive creature.

There are several attempts to bond with Sheepstealer.

Alyn Velayron (dragonseed) resulted in burns along his back

Sheepstealer proved easier to flush out, but he remained a vicious, ill-tempered beast, who killed more seeds than the three “castle dragons” together. One who hoped to tame him (after his quest for Grey Ghost proved fruitless) was Alyn of Hull. Sheepstealer would have none of him. When he stumbled from the dragon’s lair with his cloak aflame, only his brother’s swift action saved his life. Seasmoke drove the wild dragon off as Addam used his own cloak to beat out the flames. Alyn Velaryon would carry the scars of the encounter on his back and legs for the rest of his long life. Yet he counted himself fortunate, for he lived. Many of the other seeds and seekers who aspired to ride upon Sheepstealer’s back ended in Sheepstealer’s belly instead.

Silver Denys (claimed dragonseed) had his armed ripped from the shoulder and along with his sons was consumed by the Cannibal

A man called Silver Denys, whose hair and eyes lent credence to his claim to be a bastard son of King Maegor the Cruel, had an arm torn off by Sheepstealer. As his sons struggled to staunch the wound, the Cannibal descended on them, drove off Sheepstealer, and devoured father and sons alike. -The Princess & the Queen

Nettles (listed as a dragonseed in one location possibly due to the concept that she is the only character without valyrian blood to bond with a dragon)

In the end, the brown dragon was brought to heel by the cunning and persistence of a “small brown girl” of six-and-ten, named Netty, who delivered him a freshly slaughtered sheep every morning, until Sheepstealer learned to accept and expect her. She was black-haired, brown-eyed, brown-skinned, skinny, foul-mouthed, filthy, and fearless … and the first and last rider of the dragon Sheepstealer. -The Princess and the Queen

She is listed as a "dragonseed" in TWOIAF in a sidebar:

SHEEPSTEALER (Nettles): A wild dragon tamed by a dragonseed, vanished at war's end.

Whereas, unlike the other dragonseeds she looks far from a Targaryen.

But what if this bond wasn't necessary? As the dragon was pre-targaryen (just laid at a later time by the Cannibal)

• This would make A Nettle is a Leaf even cooler imo

Some other thoughts:

• This would make sense as to why the Cannibal harmed/consumed other dragons but not these other two wild dragons (even though their lairs were on different parts of the island)

For instance it was Sunfyre that killed Grey Ghost (when it was assumed to be the Cannibal):

Sunfyre went on to kill the shy, wild dragon called the Grey Ghost, leading to confused reports claiming that it was the Cannibal that did it.

• The Cannibal is always referred to as male, but:

Dragons are neither male nor female, Barth saw the truth of that, but now one and now the other, as changeable as flame. -AFFC, Samwell IV

• The Cannibal disappeared at the end of the Dance of the Dragons:

THE CANNIBAL: A wild dragon, a scavenger and killer of hatchlings, never tamed and vanished at war's end.

Why would a dragon that had no ties to the dance just "disappear" at war's end? It seems we will probably find out more in Fire and Blood II. I couldnt really come up with an answer that fit this theory except that its relatives were gone and so it left, or unless someone claimed it and then disappeared or if its currently asleep on skaagos (stone dragon).

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The alternative (that Nettles is actually a dragonseed and that some maesters (even Septon Barth and he's always right.) are correct and even the cannibal is of valyrian descent) can be argued just as logically imo.

I've always been a pretty big proponent of that idea, but I've still always struggled with the dragonseed/nettles/etc. thing.

TLDR: Due to their reactions to dragonseeds, and their behavior to each other, its possible that The Cannibal, Sheepstealer and Grey Ghost are not valyrian dragons