The term hiryū is not localized consistently in English-language versions, but is uniform in the Japanese versions. They are a separate entity from the recurring wyvern enemy species, though in the Japanese version of Final Fantasy XI, hiryū and "wyvern" are often used interchangeably, despite being separate enemy species.
In the Final Fantasy IV Setting Data Book, wyverns are referred to as the former steeds of Baron's Dragoons which, along with their riders, were phased out of service in favor of more efficient airship technology.
Wind drakes are an endangered species with only a few left in each world of the Planet R. They are nearing extinction due to the poisoning of dragon grass, which has been the phobia of wind drakes for years. Lenna and Krile both own a wind drake.
King Alexander Highwind Tycoon uses Lenna's wind drake, named Hiryu (飛竜, Hiryū?), to travel to the Wind Shrine. It gets stranded on the Northern Mountains and Lenna—along with Bartz, Galuf, and Faris—rescues him, and fly him to the nation of Walse. Hiryu can carry the party past the Tycoon Meteorite to Castle Tycoon where they can gain advanced items and equipment, and Lenna speaks with Faris about her childhood. As Hiryu cannot cross mountains the party is forced to leave it behind as they continue to Karnak.
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In Galuf's World Galuf uses Krile's wind drake to rescue Bartz, Lenna, and Faris, whom Gilgamesh is holding hostage at Castle Exdeath. He succeeds, and sends the wind drake back to Krile. Galuf does not return safely and Krile discovers, through telepathy, that Bartz, Galuf, Lenna, and Faris are in a Moogle Village, and uses the wounded wind drake to rescue them and take them back to Castle of Bal.
This makes the wind drake even more sick, and Bartz's party travels to the Drakenvale to find dragon grass and decontaminate it. The wind drake refuses to eat the plant, and Lenna eats it as a means of persuasion, but becomes sick due to the plant being poisonous to humans. The wind drake recovers, and Bartz's group uses it to travel around Galuf's World.
In the Merged World Hiryu saves Lenna from being thrown into the Void, and saves Lenna from a monster possession. Hiryu is later found atop the Phoenix Tower where it jumps off to become the Phoenix summon. Krile's wind drake is waiting atop Castle Bal and can be retrieved at any time. A wind drake later saves the four Warriors of Light from the Void.
Wind drakes controlled by the player can cross ocean, but not mountains. They can only land on plains. In the Merged World there is an Airship and Hiryuu glitch that can cause the player to lose their wind drake if they land it atop Ghido's Cave.
In the mobile and Steam versions, obtaining the wind drake earns the player the "Fear of Heights" achievement. The music that plays whenever flying a wind drake on the world map is called "Spreading Grand Wings".
The dragon, at first small and weak, soon grows before their eyes. It breaks out of the Temple of Winds and flies Prettz to the Black Moon to face off with Ra Devil. In the end, Hyriuu shrinks to its normal size once it reaches the Black Moon, and Rouge throws it to the other pirates with orders to take care of it.
Wyverns are minions summoned by the Dragoon Job, and appear as both allies and enemies.
The Wind Drakes are portrayed just like their counterparts in Final Fantasy II.
wyvern is a medieval European mythological winged creature with a dragon's head, reptilian body, two legs (sometimes none), and a barbed tail. They appear in many crests of Europe's ancient families, symbolizing strength. The word wyvern comes from wivere, which means serpent.A
Unlike the wyverns of legend, the term "wyvern" for the wind drake seems to just another name for a dragon, as they have four legs and wings.