Squid

Squid are 8-armed mobs that spawn in water. Squid are always passive towards the player, akin to the other passive mobs found on land. They can be found in water of any depth, in any biome, and spawn on any difficulty.

Uses
When killed, each squid will drop 0 - 3 Ink Sacs that can be used to dye wool. The ink sac, like a unit of dye, can be applied directly to sheep before shearing to make black wool or mixed with varying proportions of Bone Meal beforehand to yield light gray and gray dyes (that can also be used on sheep before shearing).

Behavior
When moving about, the squid's tentacles open and close, giving them an appearance of pushing themselves forward. Squid will hover in place and wander around one layer aimlessly and do not deliberately interact with the player, even if attacked.

Squid seem to be confused with light under water, they are attracted to lights like glowstone.

Squid stay alive but become immobile when beached on the land. They can take fall damage and burn like any land mob, but (due to being aquatic) they cannot drown.

You can find a video of the squid here:

History
Squid were introduced in the Beta 1.2 update.

As of Beta 1.3, squid were given the ability to move in three dimensions (rather than only along the bottom of the body of water).

As of Beta 1.5, squid are incapable of swimming upward. When they do manage to swim upward, they drift down with a zero net gain in altitude, resulting in a jittery animation. This seems to have been fixed as of Beta 1.7.

As of Beta 1.8 Pre-release 1, squid are the only mob that doesn't drop an Experience Orb when killed by the player. This has been since fixed in 1.8 Pre-Release 2.

Bugs

 * Despite not having feet, Squid make footstep sounds when swimming across blocks.
 * When logging into a multiplayer server, Squid may appear above the clouds. However, they will later disappear.
 * During multiplayer lag, mobs' positions are not updated by the server, causing especially irregular movement in Squid, such as flying and passing through blocks.
 * In RC 2, if the player stands far away, Squid will shift so that their heads point straight up and float in place until the player gets closer again.
 * When killed, Squid drop Ink Sacs, but take a second to disappear.
 * During multiplayer lag Squid are temporarily invulnerable to T.N.T.
 * During multiplayer lag, Squid will also fly up in the air if you hit them with any specified item
 * Squid, when killed, may not drop experience orbs.
 * If you look at a Squid's tentacles without the head on-screen too his tentacles do not appear.

Trivia

 * Squid are the first passive water mob to appear in Minecraft.
 * Hitting a squid underwater causes them to rise approximately 1/3 to 1/2 a block.
 * Squid cannot be damaged by attacking their tentacles (unlike Ghasts).
 * Before 1.4, using a bucket on a squid while the squid was not touching another block would yield milk.
 * Before 1.9, Squid cannot spawn in the Nether without the aid of inventory hacks, as there is no water in the Nether. As of 1.9 pre-release 4 they can spawn in water created by Ice in the Nether, and will spawn very frequently due to there being nowhere else to spawn.
 * Real squid have eight arms and two tentacles, while Minecraft squid just have eight arms, making them more akin to octopi. This may be a mis-translation due to the fact that squid and octopus have the same name in Swedish (Bläckfisk).
 * Squid behave like some land mobs and attempt to ride a Minecart. However this is not always useful as rails and water cannot occupy the same block.
 * Squid are the first passive mob that spawn at any light level, in water, and without grass.
 * When the world is loaded, all squid are aligned with the tentacles facing east.
 * It is possible to punch a squid onto land.
 * It is possible for squid to spawn in underground lakes or waterfalls due to their ability to spawn in any water, source block or not.
 * It is possible for a squid to trample a farmland block.
 * In RC2, When Squid come out of water and are hurt, they make the new player hurt sounds.
 * Squid spawners will only spawn them on the 60 depth, as the large water bodies (such as oceans, rivers and ground lakes) appear only on that depth. This means squids can no longer spawn in mountain springs or underground lakes.