Octoling

Octolings are playable characters in Splatoon 2, alongside the Inklings. Unlike Inklings, which are unlocked from the start, Octolings are only available after beating the Octo Expansion DLC.

Their Octopus form is widely different to the Octarian ones with two long tentacles that helps them move with a smaller one on top that resembles a mustache and four smaller ones on the underside along with a pair of eyes that are separated and resemble an Inkling squid's eyes. The player can switch between them and the Inklings in the Player Settings, however, the game will save both the player's Inkling and Octoling so they can play as Agent 4 or Agent 8 with the same features in their respective game mode.

Background
The playable Octolings were once Octarian fighters that abandoned the Octarian army. They are noticed by Pearl on Inkopolis Fashion Focus as "Squidkids with weird hairstyles".

The Octolings were once brainwashed by the music of DJ Octavio. However, when these Octolings hear the Calamari Inkantation, it nullifies the brainwashing effect. Many (if not all) of them defect to Inkling society in Inkopolis afterwards. This happened during the "Enter the Octobot King!" mission of the first game.

Octo Expansion
Octo Expansion introduces the mysterious Octoling character Agent 8, and will allow players to use Octolings in multiplayer upon completing its story mode. The male version of Agent 8 is also the first appearance of a character which is clearly shown to be a male Octoling in-game. Their octopus form is more reminiscent of the squid form of Inklings with a key difference being that their eyes are separated instead of together.

When the player completes Octo Expansion, they will be able to change from being an Inkling to an Octoling in the player settings. Playable Octolings have different animations for trying on gear, winning or losing, and for taking photos with amiibo. Octolings also have unique hairstyle options as well. As of Version 3.1.0 (Splatoon 2), the Octoling girl has the choice between a hairstyle featuring parted bangs with two long curlier tentacles in front, and one with a small tentacle on both sides of the face and two tentacles tied up into a ponytail. The Octoling boy has the choice between a shaved head with a single tentacle as a mohawk and three smaller ones down a column in the back, and another with a giant afro that resembles a real-life octopus' mantle that changes shape depending on headgear. The Squid Research Lab US Tumblr shows that Octolings are more serious than Inklings and have an interest in technology, e.g. instead of aiming with weapons, they will look more closely at them to see how they work; they'll wave around their phones and look at them more closely; and their icons in battles aren't seen smiling. Due to have been defecting from the Octarian army, their animations around the square make them look curious and almost confused around the place.

While both types of characters are called "Octolings" in English material about the expansion, the Japanese version makes a notable distinction between the player Octoling, called タコundefined, "Octopus" and the enemy Octolings, called タコゾネスundefined, "Octopus-Amazon."

Trivia

 * When an Octoling is splatted, they will fly off in the form of a small ghost, just like Inklings. They even have their own unique octopus ghost for this.
 * This difference extends to other parts of the GUI, such as the weapon / status indicators, splat messages, splatted enemy markers, etc. having a small cartoon octopus instead of a squid.
 * Octolings are prohibited from entering Octo Canyon. Instead, the game will automatically switch back to the previous Inkling used, customization options included, since that character would canonically Agent 4. Similarly, vise versa will happen when entering Octo Expansion as an Inkling.
 * In Marina’s model, she doesn’t seem to have any ears of sort whatsoever despite her wearing headphones most of the time. The reason for this is unknown.
 * In the art book, they were originally going to have Inkling masks and floppy ears like a dumbo octopus. The ideas were discarded except for the curly, above suckers hair and armor.
 * However, the rounded ears may have been reintroduced in Agent 8's design and the design of the playable Octolings.
 * Concept art of Salmon Run show both Inklings and Octolings fighting together against early versions of the Salmonids, suggesting that Octolings were planned to be playable from early on.
 * The playable female Octoling model appears to not be as curvy as the hostile Octoling model.
 * The playable Octolings reveal that there are male Octolings as well in the Octarian army.
 * The playable Octolings use the pink ink now in the Player Settings and the Inklings use the green ink now instead of the Inkling boy being green and the Inkling girl being pink.
 * After Octolings are unlocked, the Player Settings screen now has octopuses swimming up with the squids in the background.
 * Octolings' smartphones are different than Inklings', and have a round shape, similar to an octopus's head.
 * Octolings in the plaza have different movements than Inklings. Notably, the ones with phones show confusion when trying to use their phones and the Octoling Girl tends to demonstrate a shyer demeanor.