Vermax

Vermax was Jacaerys Velaryon's dragon from the cradle, hatched from an egg laid beside the prince at birth in the old Targaryen custom, and by the time the Dance began he had grown into a capable mount for a young man his mother needed to trust with more than ceremony. When Rhaenyra's cause required allies rather than armies, it was Jacaerys and Vermax she sent north and east to find them — a diplomatic mission by dragonback that no army could have managed half so fast. Jacaerys flew first to Winterfell, where Cregan Stark pledged the North's swords to the black cause, and on to the Vale, where the Arryns answered the same summons; a single prince on a single dragon did more to secure Rhaenyra's flanks in a few weeks than her council had managed in months of talk. The maesters note, not entirely as an aside, that no other rider of the war put a dragon to better use than this.

That use ran out at the Gullet. When ninety Triarchy warships fell on the Velaryon fleet guarding the narrow sea approach to King's Landing, Jacaerys flew Vermax into the single bloodiest naval action the histories record, dragonfire and shipfire tangled together over water thick with wrecks. Scorpion bolts loosed from the decks below found their mark, and Vermax went down into the Gullet with his rider still on his back — Rhaenyra's own heir, drowned in the sea his family had always claimed to master. The blockade held, barely, in the battle's aftermath, but the war lost its designated heir in the same afternoon, and the succession it had been fought to settle grew no simpler for it. A rumor that Vermax sired a clutch of eggs at Winterfell before his death persists in some tellings; the Citadel files it, as it files most such tales, under wishful invention rather than record.