Siren
Sirens, according to Greek mythology, were the daughters of Aqueloo and a Muse. They are creature half woman and half bird. They had a very melodic and smoothing voice that no sailor could resist them, they could challenge the Muses in a singing contest.
Here, the Muses emerged victorious, and because of that sirens started have their precious bird wings. Sirens, after such a humiliation decided to flee to the coast of Sicily, where it is said that even reside. To avenge the affront of the Muses, they attracting sailors circulating in that area, they sing a beautiful songs. The sailors who hear this song could not avoid hearing, and approach them to die while the ship crashing against the rocks near the island. In our culture when we speak of a siren normally imagined with a creature where the top of his body is a woman and bottom is a fish like mermaid.
Siren appear as a very useful GF in Final Fantasy VII, with it's technique "Silent Voice" will give the enemy bad status for a long time of period.
Valefor
Valefor is one of the Aeons in Final Fantasy X. The names often related with evil or demon story, where he was described as "Hellish Grand Duke" and according to the stories, Valefor ruled 36 legions in the hell.
There are some theory about his appearance, some story mentioned that he shaped like a fallen angel, others mentioned Valefor had a lion body and human head. There is also a third theory which mention that Valefor had goose head and legs, body and tail of a lion hares. Curiously none of these three definitions coincide with what appears in Final Fantasy X, but it is true that his appearance in the game has an appearance of a demon.
Valefor is the protector of thieves and helps them steal. It also said that Valefor is know both of the events of the past and will happen in the future.
Phoenix
The phoenix is a mythical creature mentioned in lots of different mythology, Phoenix known as a giant bird who receive different names depending on which culture is cited. These was various start from FĂȘng-Huang (China) until Benu (Egypt), or Ho-oh (Japan) and Yel (North America). At its origin, Phoenix was a small bird that lived in Eden. Phoenix was the only creature who did not want to taste the forbidden fruit, overcoming temptation. Unfortunately a spark came to a small nest and the bird burned, but as a reward for overcoming the temptation banned was granted immortality.
Phoenix emerged from the ashes like a giant bird of unmatched beauty and singing. In appearance, the phoenix resembles an eagle but ten times larger, is involved in fire and its reddish plumage is so beautiful that all animals were creating envy him. This magical creature symbolizes immortality, regeneration and ressurection. Some say that Phoenix lives during hundred years, and then incinerated in their own nest for a new Phoenix reborn from the ashes, others say it is able to live up to 1461 years, some even claim that during live 12954 years. Other theories for the regeneration of bird ensures that it dies steeped in his own semen and that the new Phoenix is responsible for carrying the body of his father to the holy city of Heliopolis where it is incinerated.
In Final fantasy series phoenix became the powerful summon to attack enemy and healing the party as well. In Final Fantasy VIII you will need an Phoenix Pinion to call him, he able to appear randomly when the party were annihilated and ressurect all party member at once.
Gilgamesh
In Sumerian Mythology, Gilgamesh known as a legendary hero. He is a son of Uruk Emperor (Lugalbanda), who married a wise and beauty Goddess named Nisun. And just like Hercules he became half Human and half God that have a very great power. After his father die, he become the next King, but he couldn't manage his kingdom as a king. So the people of Uruk ask help from a God (Anu). Anu then ask a help from another God (Aruru) and then Aruru create a powerful human, Enkidu.
When Enkidu enter Gilgamesh kingdom, Gilgamesh ask him to fight, but after a long period there's no person who win, they agree to make a peace, then they become a best friend, and rule Uruk kingdom wisely. In Final Fantasy V Gilgamesh appear as boss battle that must defeated several time. Also in Final Fantasy VIII he take place of Odin Position when Odin killed by Seifer in Lunatic Pandora.
Gilgamesh also appear in Final Fantasy IX but only as a non playable character. He appear at the beginning of the game as Alleway Jack, and teach Vivi how to play card game for the first time. He will tell you his real name if you already mastered in Card Game.
Quetzalcoatl
Along with Shiva Quetzalcoatl is the first Guardian Force (GF) owned by Squall in Final fantasy VIII he attack all of the enemy with his most powerful attack named Lightning Storm. the name Quetzalcoatl (mean Feathered Snake) was taken from a name of a god in Aztec mythology. He is a son of Virgin Goddess Coatlicue, that known as The Wise God of the Sky and also the god who create the earth.
Depend on Aztec Mythology, the universe have a lot of period on it's development, and Quetzalcoatl is a god that will create all human on every period an will spread them all over the world. First he will take the Bone and the blood from the human on one period and then he created the human from the blood and bones on the next period. Because of his duty he known as the god of the Death and the god of Resurrection. Quetzalcoatl also known in Toltects as lord of Tula, and After he death he turn into a planet near the earth which is Venus.
Fenrir
Fenrir or fenris is a name of a wolf that appear in Norse Mythology, which as a Wolf that predicted will kill every God that he meet in God Annihilation (Ragnarok). He is son of God of Evil (Loki) with a Frost Giantess (Angrboda). Because the god afraid to him, even Odin didn't dare to face him directly, he then kidnapped and taken to the Asgard. In Asgard sealid with Gleipnir ( A rope that made by the Dwarf from farious strong materials), maybe the rope looks very brittle but the rope was absolutely strong to seal Fenrir.
But to seal Fenrir, a God named Tyr must sacrifice his hand to seal this wolf, to prove that the seal wasn't dangerous for Fenrir. He sealed on a stone and his mouth also sealed with a piece of sword to make sure that he wouldn't bit anyone else. For thousand of years only Tir who dare and allowed to get near him. But Fenrir finally free at Ragnarok and kill a lots of God, and his first victim was Odin, the lord of the God. In Final Fantasy Series he appear as one of Eidolons in Final Fantasy IX, also become one of the summon and also mobs in Final Fantasy XI.
Typhoon
There are a lot of horrible creature in every Mythology. One of them is Typhoon, a giant creature shaped like a giant snake that able to shot fire from his eyes. Depend on the stories, he nominate himself as the king of the world because his great power. But there is one God that able to beat him which is Zeus. With his Lightning weapon Zeus beat him and became the ruler of the god. Zeus than take him to a prison below Mountain of Etna in Sisilia to make sure that he won't disturb anyone else. But before he lost he create a monster that have same attitude, Evils, and also has a great power. That monster is Chimera.
In Final fantasy series Typhoon appear as summon in Final Fantasy VII, but in this game Typhoon appear in a cute face and far from horrible shape.
Chaos (Chaoz)
Chaos same like Titan also appear in Greek Mythology. Long time before earth and universe created, the things that only exist is darkness. At that time the ruler of universe is Chaos, The God of the Darkness. There is no creature of even god dare to face with him, because mentioned that he has a very horrible face. After he maried, he have some child that have similar attitude like him. Only one of his child different which is Gaea. After Gaea matured, Gaea eliminate his father domination, and then he created Earth and Universe and all of living things.
Chaos appear in most of RPG game, and he always appear in a evils attitude with a very great power. In Final fantasy series he appear as Final Boss in Final Fantasy III and even Chaos became Vincent Valentine last Limit Break.
Chaos in Old Greek Myth:
Ixion
In Greek Mythology, mentioned that Ixion is son from God of War, Ares. In every myth stories he appear as a bad person with his various bad attitude. He married with Dia, and in one of the story he trapped Dia's father because denied to give him a kind of money and riches. He take his Dia's father into "Eiones" which is some kind of hole with fire surrounding it. He must be have hard punishment from Zeus but Zeus forgive him because actually Zeus fall in love with Dia, Ixion's wife.
But after dare to disturb Hera (Zeus's wife) Zeus finally give him a punishment which is placed him in Tartarus (Deepground-Hell). In Final Fantasy series Ixion appear as on of powerful Aeon in Final Fantasy X also become one of the greatest Esper in Final Fantasy XII .
Behemoth
One of monster that often appear in Final Fantasy Series is Behemoth. Behemoth appear like a bull that have purple colored of all it's skin, have a great horn and also a little wing shaped in his back. This monster have a great power so you will be difficult to beat it if you don't have enough level.
But did you know that information about Behemoth was found on Bible ? Depend on the reference in the Bible, Behemoth is the largest creature in the world that created by the God in Sixth day world creation. And on another story Behemoth often related with Bahamut. The tales say that this two creature always fight and God punish and destroy them to save other live. They also mentioned will come back at Judgment Day or Ragnarok to fight one more time until the end. Btw, the word Behemoth was taken from Hebrew word (b'hemmah) that mean Beast.
Leviathan
This creature is famous because known in some of Mythology. Leviathan known as the God of the Sea by Japanese. Leviathan (or Lotan ) also mentioned in Canaanite myth that known as evil dragon with seven head that killed by Baal. In Ugaritic Myth, Leviathan known as God of the Evil. Leviathan also mentioned in Bible as a Sea creature that shaped like a giant snake. From the myth known he have a very solid skin that able to destroy any kind of weapon.
Leviathan have a lightning eyes so he able to see in the most darkest side of the sea. Because his appearance always start with the Evil aura, Leviathan mentioned as one of Anti-Divine symbol. In Final Fantasy series Leviathan appear as powerful Summon, with his "Tidal Wave" skill that will drowning all enemies into the end of the ocean.