Concept artwork from Final Fantasy IV.
The Octomammoth (オクトマンモス, Okutomanmosu?), also known as the Octomamm, is a recurring creature in the Final Fantasy series, first appearing in Final Fantasy IV. As its name implies, it appears as a giant octopus.
Appearances
Final Fantasy IV
The Octomammoth, also known as the Octomamm, appears as a boss encountered by Cecil, Rydia, and Tellah in the Underground Waterway. It is vulnerable to Dark and Lightning.
Final Fantasy XIV
The Octomammoth appears as a boss encountered in 
The Aetherfont. Another creature localized as Octomammoth can be fished from the Limsa Lominsa Lower Decks, though this is known as the "Mega Octopus" (メガオクトパス, Megaokutopasu?) in the Japanese localization.
Final Fantasy Crystal Chronicles: My Life as a King
This section about an enemy in Final Fantasy Crystal Chronicles: My Life as a King is empty or needs to be expanded. You can help the Final Fantasy Wiki by expanding it.Pictlogica Final Fantasy
The Octomammoth from Final Fantasy IV appears as a boss.
Final Fantasy Record Keeper
The Octomammoth from Final Fantasy IV appears as a boss.
Final Fantasy Brave Exvius
The Octomammoth appears as a boss encountered in the Twelve Types Takedown, representing the Aquatic type.
Etymology
The numeral prefix "octo-", from Latin for the number eight.
A mammoth is any species of the extinct genus Mammuthus, one of the many genera that make up the order of trunked mammals called proboscideans. The various species of mammoth were commonly equipped with long, curved tusks and, in northern species, a covering of long hair. They lived from the Pliocene epoch (from around 5 million years ago) into the Holocene at about 4,000 years ago, and various species existed in Africa, Europe, Asia, and North America. They were members of the family Elephantidae, which also contains the two genera of modern elephants and their ancestors. Today, the word "mammoth" is often used to describe abnormally large animals or objects.