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Ah this is very cool Tia! I love mythology! So I am seriously finding this interesting!

I love mythology. I'm actually taking a class on it this semester, it's hella fun.

Anyway, let's see what I can come up with. *Rubs hands*

-Thor, as we probably all know, was the Norse god of thunder.
-Cybele was an earth goddess. Phrygian, according to Wikipedia, but she's roughly equivalent to the Greek Gaia.
-Atalanta, just as she's described in the games, was an expert huntress. She's from Greek mythology. As a side note, she took a vow of celibacy as part of her devotion to Artemis.
-The Nereids were Greek sea nymphs.
-Tiamat was one of the early Babylonian goddesses, who fought against Marduk.
-Kirin is probably a westernization of Qilin, a Chinese chimera-like creature wreathed in flames.
-Procne was a minor character in Greek mythology.
-Neptune was the Roman name for Poseidon, the Greek god of the sea and earthquakes.
-Zagan is one of the kings of Hell in the Ars Goetia.
-Megaera and Tisiphone were two of the Erinyes in Greek mythology.
-Clotho, Atropos, and Lachesis were the Greek Fates. Clotho spun the cloth of a person's life, Lachesis measured it, and Atropos cut it when they died.
-Ulysses was an alternate name for Odysseus, the hero of the Odyssey.
-Haures is another demon of the Ars Goetia.
-Coatlicue was an Aztec goddess.
-Daedalus is probably rather well-known, from the story of Icarus, the boy who made wings out of wax and feathers and flew too close to the sun, at which point the wax melted and he fell to his death. Daedalus was his father. Greek.
-Charon's portrayal in game is also quite accurate to his mythological persona: the boatman of the River Styx. Dead spirits would have to pay him to gain passage across the river so they could move on to their eternal rest.
-Iris stays pretty true to her Greek mythological roots too, as a minor goddess of rainbows. She was also a messenger.

That's all the summons I knew off the top of my head or could find anything significant on with a quick search, plus a few of the weapon. I know there's more out there though.

Hmm, let's see what I can do to help...

Dullahan is a powerful fairy from Irish folklore, and is essentially the Irish version of the Grim Reaper. It takes the form of a horseman who carries his head under his shoulder. This head has tiny, constantly flitting black eyes, a hideous grin that goes from ear to ear, and flesh the color and texture of moldy cheese. Often, the horse itself is headless, and the dullahan may drive a coach pulled by several such headless horses. Gates cannot stop the dullahan's journey--they just unbar automatically as it approaches. Once the dullahan stops riding, it calls out a name. The mortal who is named dies, and the dullahan drags his/her soul to the afterlife. Those who try to spy on the dullahan riding across the country are rewarded with a bucketful of blood splashed in their faces. Curiously, dullahans are terrified of gold--even a gold pin can drive them away from their task.

Deadbeard (Talos in Japan) Talos comes from a figure from Greek mythology, who was a giant bronze man/god who sunk ships approaching Crete by tossing rocks at them.

Kraken are mythical sea monsters of gargantuan size, said to have dwelt off the coasts of Norway and Iceland. The sheer size and fearsome appearance attributed to the beasts have made them common ocean-dwelling monsters in various fictional works. The legend may actually have originated from sightings of real giant squid that are variously estimated to grow to 13–15 m (40–50 ft) in length, including the tentacles. These creatures normally live at great depths, but have been sighted at the surface and reportedly have "attacked" ships.

Valukar (Balrog in the Japanese version) The huge fiery demon from Lord of the rings xp

Azul (means blue in spanish) is modeled after the Leviathan described in the Old Testament of the Bible. The name "Leviathan" is used five times in the Bible, particularly in the Book of Job where the description in chapter 41 causes many to interpret it as a massive sea serpent, which lead to its popular modern-day image.

Manticore is a mythical creature often depicted with a man's head, a lion's body and the tail of either a dragon or a scorpion. Its name means "man-eater", referring to its habit of devouring humans found in small groups in the desert. Other aspects of the creature vary from story to story; in some cases, it can shoot poisonous spines. Origin: Persia

Hydra in Greek mythology was an ancient serpent-like water beast that had multiple heads and toxic breath. The Hydra of Lerna, which was slain by Hercules for his Twelve Labors, lived in the Spring of Amymone. Its blood was then used by Hercules to make poisoned arrows for future tasks.

Whew, that should cover most of the bosses. Thank god for Wikipedia.
If I have the time and intrest I might try to make same kind of a list about legendary weapons.

Here's the weapons I gathered:

Kusanagi/Cloud brand Kusanagi-no-Tsurugi is a legendary Japanese sword as important to Japan's history as Excalibur is to Britain's, and is one of three Imperial Regalia of Japan. It was originally called Ama-no-Murakumo-no-Tsurugi ("Sword of the Gathering Clouds of Heaven") but its name was later changed to the more popular Kusanagi-no-Tsurugi ("Grass Cutting Sword").
Related to Susanoo, Kushinada, Izumo and the eight-headed serpent story

Phaeton's blade In Greek mythology, Phaëtōn or Phaethōn was the son of Helios. Perhaps the most famous version of the myth is given us through Ovid in his Metamorphoses (Book II). Phaeton seeks assurance that his mother, Clymene, is telling the truth that his father is the sun god Helios. When Phaeton obtains his father's promise to drive the sun chariot as proof, he fails to control it and the Earth is in danger of burning up when Phaeton is killed by a thunderbolt from Zeus to prevent further disaster.

Tisiphone and Vengeance Tisiphone (Ancient Greek: "avenging murder") was one of the Erinyes or Furies, and sister of Alecto and Megaera. She was the one who punished crimes of murder: parricide, fratricide and homicide. A myth recounts how Tisiphone fell in love with Cithaeron, and caused his death by snakebite, specifically, by one of the snakes from her head. In Book VI of Virgil's Aeneid, Tisiphone is recognized as the furious and cruel guardian of the gates of Tartarus.

Masamune and Muramasa A legend tells of a test where Muramasa challenged his master, Masamune, to see who could make a finer sword. They both worked tirelessly and eventually, when both swords were finished, they decided to test the results. The contest was for each to suspend the blades in a small creek with the cutting edge facing the current. Muramasa's sword, the Juuchi Yosamu (10,000 Cold Nights) cut everything that passed its way; fish, leaves floating down the river, the very air which blew on it. Highly impressed with his pupil's work, Masamune lowered his sword, the Yawarakai-Te (Tender Hands), into the current and waited patiently. Not a leaf was cut, the fish swam right up to it, and the air hissed as it gently blew by the blade. After a while, Muramasa began to scoff at his master for his apparent lack of skill in the making of his sword. Smiling to himself, Masamune pulled up his sword, dried it, and sheathed it. All the while, Muramasa was heckling him for his sword's inability to cut anything. A monk, who had been watching the whole ordeal, walked over and bowed low to the two sword masters. He then began to explain what he had seen.
"The first of the swords was by all accounts a fine sword, however it is a blood thirsty, evil blade, as it does not discriminate as to who or what it will cut. It may just as well be cutting down butterflies as severing heads. The second was by far the finer of the two, as it does not needlessly cut that which is innocent and undeserving."
It has also been told that once drawn, a Muramasa blade has to draw blood before it can be returned to its scabbard, even to the point of forcing its wielder to wound himself or commit suicide. Thus, it is thought of as a demonic cursed blade that creates bloodlust in those who wield it.

Excalibur Excalibur is the legendary sword of King Arthur, sometimes attributed with magical powers or associated with the rightful sovereignty of Great Britain. Sometimes Excalibur and the Sword in the Stone (the proof of Arthur's lineage) are said to be the same weapon, but in most versions they are considered separate. The sword was associated with the Arthurian legend very early.

Vulcan Axe
Vulcan is the Roman name for Hephaestus, a blacksmith god associated with fire and volcanoes.

Aegis Shield
Aegis is the shield used by Zeus in Greek myth.

Cronos mail - Cronos (also spelled Cronus)
Cronos was a Titan, the youngest child of Gaia and Uranus. Uranus disliked his Titan children, and when they were born he locked them in Tartarus, causing Gaia pain (for Tartarus was her bowels.) She fashioned a sickle of adamant and gave it to Cronos. When Uranus came to breed with Gaia, Cronos castrated him with the sickle and ruled in his place. He was later deposed by his own son, Zeus.

Triton's Ward - Triton
The son of Posiedon and Amphitrite. Depicted as a merman, he had a conch shell which he blew to raise or quiet the waves.

Fujin Shield
Japanese god of the wind and one of the eldest Shinto gods.

Valkyrie Mail
Characters from Norse mythology, Valkyries chose who would die in battle and brought them to the halls of Valhalla.

Erebus Armor
Erebus is the personification of darkness/shadow in Greek mythology.

Now that you mention it, yeah. It would have made more sense to give them an equivalent piece of equipment, like Einherjar Armor or something.

It's like "ayn-HARE-yar," approximately. I dunno, I'm not very good at typing out pronunciations. DX

Anyway, the reason I picked that was because the Einherjar were basically the male equivalents of the Valkyries: bold warriors who had died in battle gloriously and earned the right to fight alongside Odin at Ragnarok.