I wanted to pass along some information that Foxclon recently posted when responding to a player on the Japanese forum.

Since Nepto Dragon is a legendary fish, it is has not been included in the 106 varieties of big fish that need to be caught for the achievement “Go Big or Go Home X” in order to obtain the Ironworks fishing rod.

While we can’t share the specific value, the legendary and big fish that have been implemented can all be caught with the equipment that has been added as of Patch 2.4, and these fish can be caught without the use of free company actions when using the new equipment.