Quite a few creatures are based off of greek mythology, yes. Deal with it.

The Naga:

(The Naga are based off of the ones from WoW so that was the most appropriate image to use)

The Naga are a reptilian race of human-like serpents with a bloody history. When the world began to industrialize and spread to the Americas, the Naga which lived in the sea wondered if their home would be next for human domination. While they had no want for industry, most decided to accept that it was a possibility. Some, however, did not. A group of the Naga led by Raze, one of the strongest warriors in the dominant clan, turned on their own kind and attacked, presenting the others with an ultimatum: Fight or Die. The Naga fought back, and soon the two races were battling one another, Raze led the smaller force against the others, using underhanded tactics to tear apart the once noble people.
Chieftain Cain realized that fighting was futile. Many of his people had died or joined now Chieftain Raze, and he had little hope of defeating the sneaky naga. He led the remainder of his people away, pursued every slither of the way by his foe. It wasn't until he was approached by a strange being of shadow, who presented them a way out: An island inaccessible to humans.
When he agreed, the mystery man opened a portal and led the naga away... But the portal never closed. And Raze wasn't far behind...
The two factions of Naga on the island consist of Cain's forces, who support the mystery man and are prone to talk quicker than to fight, and Raze's forces, who live in caves scattered around the shores of the island and patrol the waters around it.
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The Automatons:
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Not everything on the island came from myths and legends. The Automatons were built by Carter Wilkes, whose boat crashed on the island in the late 1950's. Carter was experienced in building all sorts of machines, from basic pocket watches to full blown steam engines, and decided to make the most of his time on the island. Over the course of ten years on the island, not made any easier by the eventual arrival of the Naga, he built all sorts of small machines that did simple things like crack nuts, cut grass and other menial tasks. His breakthrough was a machine that could build others of its kind. Slowly, he built up a small force of hundreds of machines, constantly improving upon the base which in turn improved on the others, until he had machines that could defend their master, build a fortress, and even emulate human speech!
The three basic types of automatons that travel the island are the Tanks, the Scouts and the Harvesters.
The Tanks are bulky and hard hitters, they patrol the jungle searching for intruders like the Naga.
The Scouts are fast, flying machines that patrol the skies and point out any suspicious activity. Resembles a brass gargoyle.
The Harvesters are a strange combination of the two, fast, strong and observant. They wear hoods and masks to protect the many delicate workings underneath it, and they resemble the grim reapers. One of their arms ends in a long scythe, the other a shotgun with low ammunition. Their scythes can retract to be replaced with claws that carry their prey. They are a force to be reckoned with, but every hunter has a weakness or ten.
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The Charybdis

A sea monster that resides in a deep bool of water somewhere on the island. Rarely seen by island dwellers but known by the whirlpool its deep breaths produce.
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The Hydra

The Hydra is a beast that resides just beside the island, near a volcano near the island. It has large wings and numerous heads and is a force to be avoided not attacked. Rarely leaves the volcano, however, and is easily evaded.
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The Taraxippus

It's a ghost that freaks horses out. That's about it. OOOOOOOoooOOOooooo
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The Teumessian fox

A giant fox that roams the island randomly, rumored to be a teller of prophecy. Is destined to never be caught.
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Phoenix

Giant flaming bird. 'nuff said.
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Elementals Oh, I'm so original

There are many different types of elementals, but the four chief types are as follows:

Fire Elemental - Pyrus: The most hotheaded of the four prime elementals, he is the quickest to find a violent solution to a problem. Resides in the volcano.

Air Elemental - Wisp: Indifferent to matters on the island, she cares for her own children above all else. Constantly moving about the island.

Water Elemental - Tide: She is the most caring for all inhabitants of the island, and dislikes the other chief elementals for their lack of understanding. Resides in the whirlpool where The Charybdis lives.

Earth Elemental - Brick: Lives in the mountains with his young, keeps to himself unless threatened. The only elemental incapable of speech.

Among the four main ones are many offspring mixed with the four. Among them are Storm, Magma, Steam, and Mud, and others.

Each elemental has three basic forms: the Element they represent, a creature related to that element (Sylph, Nereid, Salamander (totally pulling out the DnD references here) or Dwarf), or a dragon. Their dragon forms are separate, however, and need to be within vicinity of the elemental to be taken over.

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The Nuckelavee
This thing is a little horrifying, just a warning to ya. This is one of the less horrifying images.

A creature that resides on the island and terrorizes all living creatures on the island, nomadic, deadly and clever. Leads to a taunting of prey, however, and usually a 'hunt' ends when it rains, as water is deadly to the beast.
Obviously dislikes water elementals.

Misc

Vinasol - A plant snake, blends in with vegetation. Chills in the Sun all day, travels through canopy of trees. Rather slow, makes travel confusing. When attacked scream and attract predators.(Hccpu)

*More will be added at a later date.

Harvesters = OP

901820 Not necessarily. They have a pretty obvious weakness, among the lesser ones (ammo, lack of rational thought provoking them to follow any form of prey no matter where they might try to go, reaction times are slower than an average human's, they don't have the thinking processes that a human does so they wouldn't be able to outsmart a sentient being.) Honestly, their only advantages are having a leg up on basic human instincts.
Oh wait, I didn't mention the obvious weakness. Oh well, I probably shouldn't give TOO MUCH away...

901939
If all else fails the universe won't let this guy die...

901976 Should I make him his own section?
THE MONTE
This bastard won't die. Rumored to have an ability to make him full.

901804
How about these two who are having a war on the island.
Laideronnette, Empress of the Pagodas
Titania, Queen of the Fairies
They look fairly similar to us outsiders. And a war between fairy-like creatures wouldn't necessarily be violent and could merely be a, 'if you talk to one side' the other side will hinder you however they can... but you know they are fairies... so more annoy than anything.

Added something that fits PERFECTLY to the island.
Ever heard of a Nuckelvee?

Made a few changes to a few things. Also, going to add a few more things in a bit