Mob A also called The Monster of the Ocean Depths was a possible answer in the 2017 Mob Vote along with The Great Hunger, the Hovering Inferno, and the winning Phantom.

It is described as a Squid-like ocean creature that sucks players down with its tentacle. The mob spawns in the Deep Ocean biome and has a large mouth or shell that it uses to propel itself through the water. Jeb says that viewers should vote for Mob A because there is currently not a lot of content in the oceans, and it would make a good challenge for players traveling across the ocean.

Mob A was eliminated after the first poll.

More facts here: https://mcmobs.fandom.com/wiki/Barnacle