Goobbue is not legacy. Goobbue could be obtained by doing a specific quest in 1.0.

Characters created on ARR can not have the Goobbue. Hence characters created on non-legacy can not have the Goobbue, even if the player is a 1.0 or a legacy player.

Give Goobue to 1.0 players in other servers.

Legacy players have a very different mount: the legacy Chocobo.