Rainmaker

Rainmaker is a Ranked Battle mode in Splatoon that was announced in the Splatoon Direct on May 7, 2015 and will be added in a future update. Nothing is known about it other than that it will somehow use the weapon shown above, which appears to be a metallic fish with a nozzle in its mouth, known in Japan as a Shachihoko.

Description
Recently, it was leaked how this mode will work:

In Rainmaker, both teams compete to snag the Rainmaker.

Once you've got it, the goal is to carry it into the other team's base! So as soon as the battle begins, grab that Rainmaker!

It's guarded by a shield, though, so you'll need to bust it out first. Once you snag it, head for the other team's base.

And be quick about it! If you run out of time or get splatted, the Rainmaker will be dropped. Its shield will also reactivate.

The Rainmaker is crazy heavy, so you can't Super Jump while carrying it. Your teammates will have to watch your back!

The Rainmaker can also shoot ink like an Inkzooka, so you can clear a path!

The closer you get to the enemy base while holding the Rainmaker, the lower your goal distance will drop.

If the battle ends without anyone carrying the Rainmaker to the goal, the team with the lowest goal distance will win!