The Big Fish is a form of transport that is used in Breath of Fire.
The ability to transform into a Big Fish is the clan ability of the Manillo. Although being a Manillo is not enough to allow transformation. A Manillo must have a Sphere in their possession in order to become the Big Fish. As a Big Fish the party is able to swim over gulches and coral, allowing access to area that would otherwise be unaccessible. While travelling on the Big Fish the party will not be attacked by monsters.
At some point before joining Ryu's party, Gobi had his Sphere confiscated leaving him unable to become a Big Fish. After learning about the Dark Dragon Clan's plan to attack Prima the Guildmaster gives Gobi the opportunity to earn back his Sphere and License, all Gobi must do is assist Ryu's party as they try to rescue Prima. Gobi agrees to the terms and accompanies Ryu. During the battle against the Dark Dragons many of the citizens of Prima become Big Fish in order to fight back. After Ryu and his friends defeat the Dark Dragons the Guildmaster returns Gobi's license and Sphere giving the party access to the Big Fish's transporting abilities.
It is unknown if Manillo retain the ability to become Big Fish in later installments of the series.