The dragons featured in HBO's House of the Dragon are crucial to the show's subtext and storytelling. Due to their ownership, House Targaryen was able to ensure its grip on the Seven Kingdoms. Therefore, the key characters in House of the Dragon are defined by their relationship to the dragons.
The Targaryens and their winged beasts marked a decadent time in the history of Westeros. There is undeniably an influx of bloodthirsty, fire-breathing dragons in the House of the Dragon, and while every one of them is magnificent in its own right, the show has established a pecking order in terms of their size.
Arrax
Arrax was the dragon ridden by Rhaenyra Targaryen's secondborn son, Prince Lucerys Velaryon. Despite his small size, he boldly fought the biggest dragon of the series, Vhagar in a fight above Shipbreaker Bay. He and his rider, Lucerys became the first casualties of the Dance.
While Aemond was intent on taking Lucerys' eye, he did not mean to kill him. The pivotal dragon vs dragon battle which began with Aemond engaging with Lucerys without any warning featured in House of the Dragon's finale episode. After a brief chase, Aemond and Lucerys completely lost control of their dragons, Vhagar killed Arrax and sent him and his rider into the sea. Their devastating and shocking deaths paved way for the Dance of the Dragons.
Vermax
Vermax who grew a lot bigger than Arrax is Rhaenyra’s firstborn son, Prince Jacaerys Velaryon's dragon. He's introduced as the Prince's mount in season1, episode 6, "The Princess and the Queen," at the Dragonpit in King's Landing. He's green and slightly taller than the order of guards who protect him.
Though Vermax is clearly bigger than Arrax, he's still smaller than other dragons featured in House of the Dragon. He and the young Prince easily bonded in episode 6 and by the end of season 1, he'd grown into a much bigger flying beast.
Dreamfyre
Dreamfyre tends to bond with young, spirited women, and was first ridden by Rhaena Targaryen and later by Helaena Targaryen. She was shown alongside Aegon II's Sunfyre in season 1, episode 7, "Driftmark."
Pale blue with silver wings, Dreamfyre was born during the reign of Aegon the conqueror. She is thus as old as the Targaryen dynasty itself. Dreamfyre is a great asset to the Greens and will get more screen time in House of the Dragon going forward.
Seasmoke
The pale, silver-grey dragon, Seasmoke was ridden by Laenor Velaryon during the War for the Stepstones. The beast proves to be pivotal in winning the climactic stage of the said conflict, i.e, the siege of Bloodstone. He is serpentine, and powerful but nonetheless younger and nimbler than the more mature dragons on House of the Dragon.
With Laenor fleeing to Essos, Seasmoke is riderless as of "The Black Queen" and resides on Driftmark. Daemon Targaryen makes mention of Seasmoke in the finale episode as an "unclaimed" dragon. The beast will undoubtedly grow bigger and fiercer with time.
Sunfyre
The most beautiful dragon to have ever lived, Sunfyre is on track to grow into a ferocious, powerful beast. The glorious dragon's gleaming golden scales shine bright like beaten gold in the sun and contrast well with his pale pink wing membranes.
Sunfrye is King Aegon II Targaryen's mount and despite his youth, is quite skilled in combat. Fans will see the Aegon ride him in the upcoming power struggle for the Throne. The beast fights valiantly but is grievously injured while battling Meleys and her rider, Rhaenys, in the battle at Rook's Rest. With one wing half torn from his body, the once-magnificent dragon is unable to fly again and spends his days healing in Rook's Rest.
Syrax
Syrax is one of the most featured and among the most powerful dragons in House of the Dragon. The yellow, green-eyed, she-dragon is named after the Valyrian goddess Syrax.
Though Sryax is not as big as Meleys, or Caraxes, she still is huge and formidable in her own right. The majestic dragon and her rider, Rhaenyra Targaryen will play a key role in the upcoming civil war. As exemplified by Rhaenyra's early labor scene with her stillborn daughter, Visenya, Syrax and Rhaenyra share a close bond with one another. As Rhaenyra suffers through childbirth, the camera sweeps between her and her anxiously roaring dragon, thereby symbolizing their close bond. Syrax is solely ridden by Rhaenyra and as a matter of fact, she doesn't take any rider after her rider's death.
Meleys
The giant she-dragon, Meleys is ridden first by Princess Alyssa Targaryen and later by Princess Rhaenys Targaryen. She's nicknamed as the Red Queen because of the scarlet scales and pink membranes on her wings. She is known to be the fastest dragon in the skies, outpacing bigger beasts such as Caraxes and Vhagar.
Meleys is not only among the bigger dragons but fearsome when roused. She's quite formidable for her age, and easily one of the most powerful beasts on the show.
Caraxes
Caraxes, aka the Blood Wyrm, is just as fascinating as his second rider, Prince Daemon Targaryen. The War for the Stepstones proves that the two make quite a powerful pair. Caraxes boasts an elongated neck and red scales and yellow eyes. Before Daemon claimed him, he was Aemon I Targaryen's mount.
Daemon uses him at a later stage in the Dance against Aemond and his beast, Vhagar in a battle at Harrenhal. Though the warfare results in the death of all four of them, Caraxes proves to be a formidable opponent against Vhagar who's twice his size.
Vermithor
Also known as the Bronze Fury, Vermithor is slated to appear in season 2. He was once ridden by King Jaehaerys Targaryen who preceded King Viserys Targaryen. He pecks right below Balerion the Black Dread in terms of size, and is currently the second-biggest living dragon on House of the Dragon.
Vermithor has huge tan wings, and is almost a hundred years old by the time the Dance of the Dragons kicks off. He is shown to be emerging from his lair in Dragonmont in season 1, episode 10, "The Black Queen" and makes Daemon look miniscule in comparison. As the second largest dragons in the show, this bronze beast plays a key role in the upcoming events of the Dance.
Vhagar
Vhagar is one of the best dragonriders, Aemond Targaryen's mount, and the biggest dragon in House of the Dragon. She is also the last remaining dragon from the days of Aegon's conquest, and therefore the oldest flying beast on the show.
Vhagar was ridden by Visenya Targaryen during the War of Conquest, and was one of the three remaining dragons who came to Westeros with the Targaryens. Vhagar has grown to be as big as Belerion, and will prove to be the greatest asset to the Greens.