Today, we're going to plumb the depths of some old-time religion, because someone has asked this question: Tell me everything you know about the Mesopotamian Bahamut myth. I will love you forever if you do. Bahamut is certainly an interesting mythological figure, but we will have to partially disappoint you by not telling you about the Mesopotamian myth... because the myth isn't Mesopotamian!
Bahamut, a vast, primordial fish, is actually a figure out of Arab mythology; Arab myth of course did have plenty of commerce with the Mesopotamian religions, but it also has unique cultural beliefs of its own, and Bahamut happens to be one of them. Some older scholarly traditions tend to refer to all Middle Eastern mythologies as "Mesopotamian" or at least to lean heavily on Mesopotamia's influence on them, but to do so leaves out important cultural context and tends to erase other cultures in the area, so let's give Arab myths, obscure and poorly-recorded as they are, their fair shake.
Like many other pre-Islamic Arab mythological features, we are pretty sure that Bahamut predates the introduction of Islam, but due to the largely oral nature of the religion at that time and the thorough efforts of Islam to eradicate it, most of our information on the greatest of all fishes actually comes to us through the work of Muslims, which in early times sometimes incorporated the creature into Islamic cosmology as well. It is described in the twelfth-century Arab geographical work Kharîdat al-'Ajâ'ib wa farîdat al-gharâib as a fish so enormous that it takes part in supporting the universe, which is organized in a series of escalating layers; the universe is supported on the shoulders of an angel, who stands upon a giant ruby, which is set upon the back of the giant bull Kujuta or al-Rayyan of four thousand eyes, which stands on Bahamut, the massive fish (and if you want to go all the way down, Bahamut's ocean is sometimes above twin abysses of air and fire, beneath which lurks the massive serpent Falaq at the bottom of the universe, where it does not swallow all of creation only because it fears Allah's wrath).
Now, all of this is old-school Arab folkloric belief; the giant cosmological creatures beneath the world were popular in the early days of Islam, probably as holdovers from pre-Islamic beliefs, and persisted into the medieval period as part of popular literature, but they are generally not part of modern Islam across the world today. Bahamut (and Kujuta and Falaq, while we're at it) does not appear in the Qur'an, but is often discussed by the many scholars and commentators who interpreted it at various points in Islam's history, some of whom were (or are) considered authorities on the religion. Since they're interpreters of the holy scripture, however, and often adding material from local folklore and religion to boot, they often disagree with one another and their cosmological additions often aren't "official" - for example, while Bahamut-as-fish is the most common version of the creature, Tafsir Ibn-'Abbas, a famous Sunni commentary, instead organizes the universe with the great fish supporting the world and then being supported by the bull below it, and calls the bull Bahamut instead of the fish.
And of course none of this applies at all to many forms of Islam, such as Sufism, which often have their own cosmologies. This is all strictly Olden Days of Yore here.
In addition to its cosmological role, the sheer vast size of Bahamut is described as being so enormous that mortal minds can't comprehend or see its size; Jorges Luis Borges, in his Manual de zoología fantástica, claims that all the all the oceans of the world would be no larger if placed in its nostril than a single mustard seed in a desert, if that helps give you any indication of the sheer size we're talking about here. Of course, whether or not you want to take Borges seriously is up to you; he's a fantastic writer of fiction but was well-known to embellish and subtly influence things he translated for his own literate aims, so while he might be totally legit here, he also might be being just a wee bit creative.
And speaking of Borges, like several other scholars, he draws a connection between Bahamut and the great fish from A Thousand and One Nights, one of the most famous compendiums of stories from the Middle Eastern world; on the 496th night, Scheherazade describes the cosmography of the world (seven worlds upon an angel upon a rock upon a bull upon a giant fish) and relates a tale in which `Isa, the Islamic form of Jesus as a prophet of the lord, requests that Allah let him see the giant fish upon learning of its existence. Allah takes `Isa to the ocean where the fish lives, but when it passes by him `Isa falls immediately unconscious; when he awakes, he says that he saw what seemed to him to be a great bull that was three days' journey long, and Allah tells him that this was only the head of the fish passing him by and that Allah can easily create things so enormous whenever he chooses, whereupon `Isa is duly impressed. The fish in this story is never directly named as Bahamut, but it isn't much of a stretch to suggest that it's probably one and the same cosmological creature.
As I'm sure plenty of you have already noticed, Bahamut resembles, in both size and name, the Hebrew legend of Behemoth, a creature named in the Book of Job and described as one of the first works of God, enormous and unconquerable. Behemoth is the subject of pretty much eternal debate among scholars from all kinds of religious and historical disciplines; some question whether the passage in Job is trying to describe a real creature that existed in the world (possibly an elephant or rhinocerous), a description of a sauropod dinosaur that had survived until this point at least in legend, some other mysterious beast that was real but that we haven't been able to discover, a religious creature or spiritual animal outside the realm of humanity, or possibly some kind of metaphor. That debate will be going on for a good many years to come, we imagine, because unlike Bahamut, which doesn't appear in the Qur'an and therefore doesn't have to be considered too deeply by modern Islam, Behemoth does appear in both the Tanakh and the Bible, so those religions have a strong interest in interpreting its meaning.
One of the interesting things here is that, while it seems that Bahamut and Behemoth probably share a linguistic root, most likely a proto-Semitic word meaning "beast" that developed in different directions for Hebrew and Arabic (an alternate theory suggests they might be borrowed from the ancient Egyptian pehemau, the reconstructed word for "water beast" meaning hippopotamus, but this theory has its skeptics), they are clearly not the same creature. The Hebrew Behemoth stands in parallel with the Leviathan, the great sea monster of the same scriptures, and is clearly intended to represent a land-dwelling animal, while the Arab Bahamut is always clearly defined as a fish or whale. Although there is a misty possibility that both cultures are referring to the same ancient beast, it's most likely that they are not, and that the similar names are an accident of etymology.
Finally, while Bahamut is not natively Mesopotamian, it's not far-fetched to believe that it might have roots in Mesopotamian mythology. The myths of Babylon, Assyria and Sumer were the most massively influential forces in the ancient Middle East and pre-Islamic Arab people frequently worshiped Mesopotamian gods alongside their own, so it's not out of the question that the myth of Bahamut might have been influenced by those religious ideas. Mesopotamian mythology is full of gigantic and dangerous sea monsters, beginning with Tiamat herself, and their focus on the ocean as the primordial source of all most gigantic and unfathomable things might have played a role in the ancient formation of the idea of Bahamut.
And after all of this... then came medieval Catholicism, and with it a host of new craziness based on various other mythologies that the church deemed evil and wanted to prevent people from worshiping. Catholic legends bring us to Bahomet or Baphomet, which is the pagan "god" that the Knights Templar were accused of worshiping and which was a large part of the accusations against them by the rest of the church and their eventual downfall and dissolution at the beginning of the fourteenth century. Descriptions of what this Bahomet looked like or represented are incredibly vague - possibly, many scholars contend, because it was an invention created to discredit the Templars rather than being an actual focus of worship for anyone - and usually say that it was worshiped as an idol that was a severed head, or sometimes a head with three faces. By the nineteenth century, the legend of Bahomet had become part of the occult landscape and was used by various writers to claim that the Templars and anyone else associated with them (the Freemasons were particularly popular targets) were secretly worshiping something much older, and that Bahomet must have predated them despite the lack of any evidence mentioning him prior to the Templar accusations. Finally, in the 1850s, the occultist Eliphas Levi published a two-volume treatise on magical rituals in which he included Bahomet, referred to him as "the Sabbatic goat", and illustrated him with what became a highly popular image in occultism:
The image stuck, and allowed later writers to conflate Bahomet with a host of older pagan traditions involving goats and horned figures, as well as to become a major image in various different occult traditions and branches of Satanism. Of course, this Bahomet has nothing at all to do with the ancient Arab and Hebrew creatures - not even similar names, since while some attempts have been made to link the three words etymologically, most of the time Bahomet is traced back to Mahomet, the French form of the name of the Muslim prophet Mohammad and most likely chosen to link the Templars to Islam - but because of its popularity in later Europe, many, many attempts to connect Bahamut, Behemoth and Bahomet have considerably muddied the water when it comes to the original mythological figures and their origins.
But, at any rate, the fish certainly became a part of Arab mythology and filtered into our popular consciousness as a result, and is among the many cosmic creatures that could be encountered on the hero's journey. Beware its great might!