Ok, so you all probably know this. But then again, this site (according to Ondori's statistics) is really a popular FFXI hang out. So, hopefully this makes someone's day.
Ok, first off you want to backup the original copy of your Moogle's file, (JIC you really screw it up and need to fix it). Your moogle is located in the following file:
(drive)/ProgramFiles/PlayOnline/SquareEnix/Final Fantasy XI/ROM/3/56.dat
(Warning: this is not an exact path since I don't know what drive you installed to, you may have to do some searching.)
Now, take 56.dat (your moogle), and copy it somewhere you can remember it. I just moved mine up a few folders. From this point, you can then move through the other folders in these directories and search for the correct file for Bahamut. I haven't really be able to find a complete listing for every model in the game, but there is a partial list over at Killing Ifrit's. It's listed under "Guides/FAQ" and "How to change your moogle". Last I looked they didn't have Bahamut's file listed yet, so you may want to do a little trial and error searching to track him down.
To do that, you just identify a file you want to try out, copy it to your desktop or some other folder, and change the name of that old .dat file to 56.dat. You then drop it into the ROM/3 directory where the old moogle file was and log on to check what changed. Be advised that some of the files are empty or full of strange data, so you may end up crashing, (hence the backing up the old moogle file we just did). If you crash, just change the moogle file again and relog, this shouldn't permanently damage any data as long as you don't do anything but check to see if the moogle has changed.
Finally, remember that this change is purely client side, anyone else logging onto your account from somewhere else and looking in your MH will see a regular moogle. The only data that has been changed is on your computer where it loads the new model instead of the moogle. Thus, if you want to change the moogle over different computers, you have to repeat the process for each new one.
Model Numbers:
3-0 ???? (Invisible)
3-1 ???? (Invisible)
3-2 ???? (Some type of Orc)
3-3 Galka NPC
3-4 Galka NPC (Same as above?)
3-5 Galka NPC (Same as above?)
3-10 ???? (Invisible)
3-25 Home Point
3-29 Telepoint Crystal
3-34 Lion
3-37 Cid
3-36 Prince Trion
3-39 Prince Pieuje
3-41 King Destin
3-43 Curilla
3-44 Cornellia
3-45 President Karst
3-49 The Star Sybil
3-52 Aldo
3-56 Moogle
3-57 ???? (Pale Man wearing Black Robes)
3-58 ???? (Invisible)
3-63 Dragon
3-103 Chocobo
4-103 Bat
4-105 Bats
4-109 Rarab
4-110 White Rarab
4-112 Bee
4-116 Bomb
4-120 Giant Scorpion
4-122 Axe Beak
4-124 Slime
4-126 Goobbue
4-127 Mandragora
5-0 ???? (Black Mandragora)
5-1 Chaos Idol
5-2 Sabertooth Tiger
5-7 Raptor
5-9 Giant Toad
5-13 Lizard
5-14 White Lizard
5-15 Dhalmel
5-19 Sheep
5-23 Pugil
5-28 Crab
5-29 Skeleton
5-38 Cactuar
5-40 Funguar
5-42 Malboro
5-46 Treant
5-47 Twinkling Treant
5-49 Sapling
5-50 Crawler
5-52 Behemoth
5-53 Behemoth (King?)
5-56 Magic Pot
5-60 Monkey
5-62 Dragon
5-66 Orcish Wallbreacher
5-70 Stone Golem
5-71 Fire Elemental
5-72 Ice Elemental
5-73 Air Elemental
5-74 Earth Elemental
5-75 Water Elemental
5-76 Thunder Elemental
5-77 Light Elemental
5-78 Dark Elemental
5-80 Akbaba
5-82 Damselfly
5-83 Weapon
5-84 Dancing Weapon
5-117 Goblin
5-120 Goblin Fisher
6-23 Cardian
7-50 Gigas (Blue)
7-60 Gigas
7-67 Demon
7-80 Shadow Lord
7-81 Shadow Lord (Again? Diffferent?)
7-95 San d'Orian Flag
7-96 Bastokan Flag
7-97 Windurstian Flag
7-98 Beastmen Flag
9-17 Orcish Brawler
9-18 Orcish Fighter
9-19 Orcish Cursemaker
26-121 (This file contains names rather than a Model)
27-0 (This file contains names rather than a Model)
27-1 (This file contains names rather than a Model)
27-2 (This file contains names rather than a Model)
27-3 (This file contains names rather than a Model)
27-4 (This file contains names rather than a Model)
97-9 Treasure Chest
97-10 Treasure Chest (Again?)
97-11 Treasure Chest (Blue)
97-12 Giant Treant (Pink Leaves?)
97-61 Earth Elemental (Odd X, Y, Positions)
97-66 Carbuncle
97-67 Fenrir
97-68 Ifrit
97-69 Titan
97-70 Leviathan
97-71 Garuda
97-72 Shiva
97-73 Ramuh
97-74 Wyvern
97-75 ???? (Hume with Blonde Hair Floating, Crystal Warrior?)
97-77 ???? (Crouching Mithra?)
97-78 ???? (Floating Tarutaru Black Mage, Crystal Warrior?)
97-81 Cyanauce
97-82 Black Gigas (Axe)
97-83 Black Gigas (Spiked Club)
97-84 White Gigas
97-85 Black Gigas (No weapon)
97-86 Carbuncle
97-88 ???? (Old Elvaan Male, wearing dark red armor)
97-97 ???? (Blue Glow?)
ROM 2
---------------------------------------
22-8 ???? (Hume Crystal Warrior)
22-9 ???? (Elvaan Crystal Warrior)
22-10 ???? (Mithra Crystal Warrior)
22-11 ???? (Tarutaru Crystal Warrior)
22-12 ???? (Galka Crystal Warrior)
22-13 Fenrir
22-20 ???? (Rank 10 Boss)
22-56 Tonberry
23-71 Nomad Moogle
ROM
3/5 = Black-haired armored Galka
3/8 = Nada
3/24 = Jeunoan Duke's brother
3/25 = Home point
3/29 = A telepoint crystal
3/34 = Lion (the chick, not the animal)
3/36 = Trion
3/37 = Cid
3/39 = Prince Pieuje
3/40 = Princess of San d'Oria
3/41 = King Destin
3/43 = Curilla
3/44 = Cornelia
3/45 = President Karst
3/46 = Little Galka from DRK flag quest
3/48 = Some Taru
3/49 = Star Sybil
3/52 = Guy with dagger on his belt
3/56 = Moogle
3/57 = Man in dark hooded robe
3/59 = Goblin
3/60 = Chocobo
3/61 = Chocobo pecking and swaying head
3/62 = Chocobo pecking at the ground
3/63 = Chocobo with slightly different animation
3/64 = Old man
3/65 = Bastokan old man with walking stick
3/66 = Another old man
3/67 = Another old man
3/68 = Old woman, sometimes the name disappears
3/69 = Same as 68 with different clothes
3/70 = Same as 68 with different clothes
3/86 = Hume miner
3/102 = Goblin
3/103 = Chocobo
3/120 = Galka
5/0 = Black Mandragora
5/1 = Golem
5/2 = Golem
5/3 = Tiger
5/4 = Tiger
5/7 = Skink
5/8 = Skink
5/9 = Blob
5/10 = Blob
5/13 = Lizard
5/14 = White Lizard (?)
5/15 = Dhamel
5/16 = Dhamel
5/17 = Vulture
5/18 = Vulture
5/19 = Sheep
5/20 = Sheep
5/21 = Big ram
5/22 = Smaller ram
5/23 = Pugil
5/25 = Sea Horror/Monk/Bishop
5/27 = Snipper
5/28 = Crab
5/29 = Skeleton
5/31 = Skeleton
5/33 = Evil dog from Gusgen
5/35 = Coerl
5/36 = Banshee
5/38 = Cactuar
5/40 = Funguar
5/42 = Marlboro
5/44 = Argus
5/46 = Treant
5/47 = Treant (?)
5/48 = Sapling
5/49 = Sapling
5/50 = Crawler
5/52 = Behemoth (You can walk thru this one)
5/53 = King Behemoth
5/54 = Beetle
5/56 = Magic pot
5/57 = Some weird floating machine
5/58 = Bigger and higher Magic pot
5/60 = Monkey
5/62 = Wyvern thing
5/64 = Worm
5/65 = Worm
5/66 = Orcish Stonelauncher
5/68 = Cardian w/pole
5/69 = Stone Golem
5/70 = Stone Golem
5/71 = Fire Elemental
5/72 = Ice elemental
5/73 = Air elemental
5/74 = Earth Elemental
5/75 = Water Elemental
5/76 = Lightning Elemental
5/77 = Light Elemental
5/78 = Dark Elemental
5/79 = Akbaba
5/80 = Yagudo
5/81 = Fly
5/83 = Weapon w/o floating weapon
5/85 = Weapon
5/87 = Weapon
5/89 = Weapon w/ hammer in his eye
5/91 = Weapon
5/97 = Weapon
5/109 = Weapon
5/120 = Goblin
6/0 -> 6/16 = Various Goblins
6/17 -> 6/31 = Various Cardians
6/32 -> 6/53 = Samurai
6/54 -> 6/69 = Various Skeletons
6/70 -> 6/93 = Various Yagudo
6/94 -> 6/127 = Orcs and Quadavs
7/56 = Gigas
7/66 = Evil looking thing
7/76 = Evil looking thing w/scythe
9/17 = Orcish Brawler
9/18 = Orcish Fighter
9/19 = Orcish Cursemaker
9/56 = Skeleton w/dagger
9/63 = Skeleton w/ cool sword
9/121 = Skeleton w/ cool sword
9/126 = Skeleton w/ cool sword
9/127 = Skeleton w/ cool sword
ROM2
22/8 -> 22/12 = Crystal Warriors
22/13 = Fenrir (OMG THIS KICKS)
22/20 = Some Paladin looking person
22/56 = Tonberry
22/118 = Big red ant thing
23/0 -> 23/5 = An ant in armor w/ various staves
23/6 -> 23/17 = A more heavily armored ant thing w/ various staves
23/61 -> 23/67 = Female Hume w/ various dresses
23/68 -> 23/70 = Male Hume w/ various robes
23/71 = Another, different Moogle
97/9 = Treasure Chest
97/10 = Treasure Chest
97/11 = Blue Treasure Chest
97/12 = Giant Treant w/ pink leaves
97/61 = Half of an Earth Elemental
97/66 = Carbuncle
97/67 = Fenrir
97/68 = Ifrit
97/69 = Titan
97/70 = Leviathan
97/71 = Garuda
97/72 = Shiva
97/73 = Ramuh
97/74 = Wyvern
97/75 = Flying Hume w/ Paladin AF
97/77 = Crouching Mithra w/ kinda Warrior AF
97/78 = Flying Taru w/ Black Mage AF and Scythe
97/81 = Cyanauce
97/82 = Black Gigas w/ Axe
97/83 = Black Gigas w/ Spiked Club
97/84 = White Gigas
97/85 = Black Gigas unarmed
97/86 = Carbuncle
97/97 = Untargetable blue glow
Credits: Because I'm lazy, I copy/pasted it all from ffxionline.com (search out "moogle" - one of those threads) and killingifrit.com's faq/guide for the list of dat files.
Personally, Fenrir is my moogle ^^. Enjoy.