Octomaw

The Octomaw is the last Great Octoweapon battled in Octo Valley.

Appearance
The Octomaw is a giant metallic Zapfish-like Octoweapon. Its head appears to be made of wire with its body made of scrap quickly put back together. According to The Art of Splatoon, it has two frog-like feet despite being based on a fish. It has a large fin that pokes out of the ground when swimming towards Agent 3. Its eyes are expressive: they curve to form sad eyes after it fails to eat Agent 3 and they form "X" shapes when a bomb is thrown into its mouth.

Background
The Octomaw was an Octoweapon created during The Great Turf War by the Octarian army. When victory seemed to be upon the Octarians, a plug was pulled and caused the Octomaw and the rest of the Octo Weaponry to shut down. Nearly 100 years later the Octomaw was revived by a Zapfish to charge once again.

Octo Valley
Agent 3 encounters it inside a Boss Kettle in Sector 4. When it sees Agent 3, it will charge and try to eat them from below. Agent 3 must escape its maw by destroying its teeth surrounding them, which do not regenerate for the rest of the phase. Once the Octomaw pops out completely, Agent 3 can throw a bomb (preferably a Splat Bomb or Burst Bomb) into its mouth to stun it, making it reveal its tentacle which must be splatted. If time passes without Agent 3 throwing a bomb into its mouth or splatting its vulnerable tentacle and it has remaining teeth, the Octomaw will return to its initial position and launches its remaining teeth toward Agent 3 that float midair and fire globes of ink (similar to those fired by Octotroopers). The teeth can still be destroyed in this state and the remaining ones will eventually return to the Octomaw, who will then charge at Agent 3 to eat them again. If it has no teeth, it skips this teeth-launching attack and attempts to eat Agent 3 immediately.

In the second phase, it has stronger purple teeth that have more health. In the third phase, it has even stronger golden teeth.

After its tentacle is splatted for the third time, Octomaw will be defeated and blows up, granting Agent 3 its Zapfish.

Trivia

 * The Octomaw appears to have been roughly reinforced with scrap metal, suggesting that the machine has been hurriedly repaired since its last defeat.
 * If Agent 3 stuns the Octomaw by feeding it a Seeker, an exclusive dialogue of Agent 1 and 2 will be shown, with the former praising Agent 3 for having a "good thinking" and the latter suggesting them to use "ordinary bombs" (i.e. Splat Bombs or Burst Bombs).
 * Throughout the stage several signs urge Agent 3 not to feed the Octomaw a Splat Bomb, which hints at how to defeat the boss.
 * There are Easter eggs in boss missions. After killing a boss, a mysterious noise that plays in the background when players don't want to collect the Zapfish yet, even after the boss's defeat. The mysterious noise doesn't appear after defeating the boss in this mission.
 * The Boss Salmonid, Maws, has a similar attack and can be fed a Splat Bomb to splat it.