From there the skulls ranged upward in size to the three great monsters of song and story, the dragons that Aegon Targaryen and his sisters had unleashed on the Seven Kingdoms of old. The singers had given them the names of gods: Balerion, Meraxes, Vhaghar. Tyrion had stood between their gaping jaws, wordless and awed. You could have ridden a horse down Vhaghar’s gullet, although you would not have ridden it out again. Meraxes was even bigger. And the greatest of them, Balerion, the Black Dread, could have swallowed an aurochs whole, or even one of the hairy mammoths said to roam the cold wastes beyond the Port of Ibben.

- Book 1: A Game of Thrones

The hosts met on the broad plains of the Reach, amidst golden fields of wheat ripe for harvest. When the Two Kings charged, the Targaryen army shivered and shattered and began to run. For a few moments, the chroniclers wrote, the conquest was at an end . . . but only for those few moments, before Aegon Targaryen and his sisters joined the battle. It was the only time that Vhaghar, Meraxes, and Balerion were all unleashed at once. The singers called it the Field of Fire. Near four thousand men had burned that day, among them King Mern of the Reach. King Loren had escaped, and lived long enough to surrender, pledge his fealty to the Targaryens, and beget a son, for which Tyrion was duly grateful. 

- Book 1: A Game of Thrones

“Aegon’s dragons were named for the gods of Old Valyria,” she told her bloodriders one morning after a long night’s journey. “Visenya’s dragon was Vhagar, Rhaenys had Meraxes, and Aegon rode Balerion, the Black Dread. It was said that Vhagar’s breath was so hot that it could melt a knight’s armor and cook the man inside, that Meraxes swallowed horses whole, and Balerion . . . his fire was as black as his scales, his wings so vast that whole towns were swallowed up in their shadow when he passed overhead.” 

- Book 2: A Clash of Kings

Of the five dragons who had flown with Aenar the Exile from Valyria, only one survived to Aegon’s day: the great beast called Balerion, the Black Dread. The dragons Vhagar and Meraxes were younger, hatched on Dragonstone itself.

- Fire and Blood "Aegon's Conquest"

Rosby yielded to Rhaenys and golden-eyed Meraxes without a fight.

- Fire and Blood "Aegon's Conquest"

Yet it was said that Rhaenys spent more time on dragonback than her brother and sister combined, for above all things she loved to fly. She once was heard to say that before she died she meant to fly Meraxes across the Sunset Sea to see what lay upon its western shores.

- Fire and Blood "Aegon's Conquest"

Queen Rhaenys accompanied him, astride Meraxes of the golden eyes and silver scales.

- Fire and Blood "Aegon's Conquest"

Lords Errol, Fell, and Buckler hid in their familiar forests until Queen Rhaenys unleashed Meraxes and a wall of fire swept through the woods, turning the trees to torches.

- Fire and Blood "Aegon's Conquest"

One hill fell, and then another, and the fourth and final charge of the Storm King and his knights broke through the Baratheon center…only to come upon Queen Rhaenys and Meraxes. Even on the ground, the dragon proved formidable. Dickon Morrigen and the Bastard of Blackhaven, commanding the vanguard, were engulfed in dragonflame, along with the knights of King Argilac’s personal guard. The warhorses panicked and fled in terror, crashing into riders behind them, and turning the charge into chaos. The Storm King himself was thrown from his saddle.

- Fire and Blood "Aegon's Conquest"

For a few days it was feared that Storm’s End might suffer the same fate as Harrenhal, for Argilac’s daughter Argella barred her gates at the approach of Orys Baratheon and the Targaryen host, and declared herself the Storm Queen. Rather than bend the knee, the defenders of Storm’s End would die to the last man, she promised when Queen Rhaenys flew Meraxes into the castle to parley. “You may take my castle, but you will win only bones and blood and ashes,” she announced…but the soldiers of the garrison proved less eager to die. That night they raised a peace banner, threw open the castle gate, and delivered Lady Argella gagged, chained, and naked to the camp of Orys Baratheon.

- Fire and Blood "Aegon's Conquest"

When Torrhen Stark reached the banks of the Trident, he found a host half again the size of his own awaiting him south of the river. Riverlords, westermen, stormlanders, men of the Reach…all had come. And above their camp Balerion, Meraxes, and Vhagar prowled the sky in ever-widening circles.

- Fire and Blood "Aegon's Conquest"

Where the Greenblood met the sea, Rhaenys came upon the Planky Town, where hundreds of poleboats, fishing skiffs, barges, houseboats, and hulks sat baking in the sun, joined together with ropes and chains and planks to make a floating city, yet only a few old women and small children appeared to peer up at her as Meraxes circled overhead.

Finally the queen’s flight took her to Sunspear, the ancient seat of House Martell, where she found the Princess of Dorne waiting in her abandoned castle.

- Fire and Blood "Aegon's Conquest"

The start of the First Dornish War is generally fixed at 4 AC, when Rhaenys Targaryen returned to Dorne. This time she came with fire and blood, just as she had threatened. Riding Meraxes, the queen descended out of a clear blue sky and set the Planky Town ablaze, the fires leaping from boat to boat until the whole mouth of the Greenblood was choked with burning flotsam, and the pillar of smoke could be seen as far away as Sunspear. The denizens of the floating town took to the river for refuge from the flames, so fewer than a hundred died in the attack, and most of those from drowning rather than dragonfire. But first blood had been shed.

- Fire and Blood "Reign of the Dragon: The Wars of King Aegon I"

And Rhaenys and Meraxes returned once more to the Hellholt…where tragedy struck. The Targaryen dragons, bred and trained to battle, had flown through storms of spears and arrows on many occasions, and suffered little harm. The scales of a full-grown dragon were harder than steel, and even those arrows that struck home seldom penetrated enough to do more than enrage the great beasts. But as Meraxes banked above the Hellholt, a defender atop the castle’s highest tower triggered a scorpion, and a yard-long iron bolt caught the queen’s dragon in the right eye. Meraxes did not die at once, but came crashing to earth in mortal agony, destroying the tower and a large section of the Hellholt’s curtain wall in her death throes.

Whether Rhaenys Targaryen outlived her dragon remains a matter of dispute. Some say that she lost her seat and fell to her death, others that she was crushed beneath Meraxes in the castle yard.

- Fire and Blood "Reign of the Dragon: The Wars of King Aegon I"

Sixty years old, his health already failing, the new Dornish prince had no appetite for further slaughter. He began his reign by sending a delegation to King’s Landing, to return the skull of the dragon Meraxes and offer King Aegon terms of peace. His own heir, his daughter Deria, led the embassy.

- Fire and Blood "Reign of the Dragon: The Wars of King Aegon I"

“Only one answer makes sense. Recall, if you will, that Balerion was the largest and oldest of the three dragons that King Aegon and his sisters rode to conquest. Vhagar and Meraxes had hatched on Dragonstone...."

- Fire and Blood "Jaehaerys and Alysanne: Their Triumphs and Tragedies"

Morion had been weaned on the tales of past Dornish glory, and like many young Dornish lords he had seen the sun-mottled bones of the dragon Meraxes at the Hellholt. Every ship in his fleet was therefore manned with crossbowmen and equipped with massive scorpions of the sort that had felled Meraxes. If the Targaryens dared to send dragons against him, he would fill the air with bolts and kill them all.

- Fire and Blood "The Long Reign - Jaehaerys and Alysanne: Policy, Progeny, and Pain"