Balerion

Balerion the Black Dread was born in the Freehold of Old Valyria and crossed the Narrow Sea with Aegon Targaryen and his sisters to Dragonstone. His scales were black as midnight, so were his horns, his tongue and his eyes, and the flames he breathed ran red-black — a heat that could melt stone and steel alike. No dragon before or since has matched his size; his wingspan was said to blot out the sun.

He carried Aegon through the Conquest of Westeros. With Vhagar and Meraxes he burned Harren the Black and his granite towers of Harrenhal to nothing, and at the Field of Fire he and his sisters annihilated the combined armies of the Reach and the Westerlands. Where Balerion flew, kingdoms bent the knee: Torrhen Stark knelt rather than face the black dread, and Dorne alone refused the dragon and paid for it in fire.

Balerion lived two hundred years, long enough to outlive even Aegon the Conqueror, who rode him last of the great kings. He died in 94 AC in Oldtown, and his blackened skull — still large enough to swallow a hound — is kept beneath the Red Keep. Every Targaryen dragon since has been measured against him, and in the far east men whisper that Daenerys Targaryen's Drogon is the Black Dread reborn.