It's easy to watch Game of Thrones on HBO or read the books and focus on the people, not the monsters. Most fans consider the various houses to be the main players of the series, but as Ned Stark used to say, "Winter is coming," and with it, some seriously dangerous legendary creatures.
Other than the various factions fighting for the throne, the White Walkers are bringing an army south of the Wall and things are bound to heat up. There are numerous creatures within the GoT world, but these represent the most powerful of them all, ranked from weakest to strongest.
A race of giants once existed north of the Wall, but they are believed to have gone extinct with the death of Mag the Mighty at the Battle of Castle Black. Giants spoke their own language and had their own society, but were considered mythical to the people south of the Wall.
Legend has it that Bran the Builder worked alongside the Giants to help raise the Wall. While this may be true, it is further believed that he might have used magic to enslave them for this purpose. The only remaining Giants have risen from the dead as Wights.
The Children of the Forest are some of the most mysterious of all the legendary creatures in Westeros. Legend has it, they were the first inhabitants of the region said to have settled the land some 12,000 years before Robert's Rebellion. They are called the Children of the Forest due to their childlike appearance, even after maturing into adulthood.
They live for an incredibly long time, their hands have only four fingers, they have a respect for nature, which they worship, and are able to use magic, but there isn't much more known about them. As far as anyone knows, they went extinct long ago.
Wights are the undead army of the White Walkers. They are incredibly dangerous, seeing as their recruitment campaign is so effective. Once they kill someone, that person is then resurrected to join the ranks. The White Walkers appear to have the ability to reanimate the dead regardless of their species, though the majority of their horde consists of former humans.
Like other zombies from fiction, they aren't as easy to "kill" as they should be. Chopping them up won't stop them, but they are vulnerable to fire. Attacking them with flaming weapons will put them down permanently, which gives hope to any army equipped with a dragon or two.
Shadows are demons born of darkness that are created or brought into the world via the Lord of Light. They can be called upon to undertake dangerous missions or carry out assassinations. A Shadow was borne from the union of Stannis Baratheon and Melisandre for the sole purpose of killing Renly Baratheon.
Shadows are somewhat ethereal shadowy beings capable of intangibility. They are able to form their bodies into solid shapes, which can be used as weapons to wreak havoc among the living. When their tasks are complete, a Shadow ceases to exist and returns to whatever evil plain from which it originated.
The Deep Ones are a legendary race of humanoid beings who are believed to have existed in ancient times during the Dawn Age. It is believed that they were a mix of man and fish that lived in the depths of the oceans. They came up to the coastal waters to bother the humans living there.
The Deep Ones are most closely associated with the myths and legends of the Iron Islands, but it is unknown if they truly existed. There are legends across the whole of the known world of similar beings so they likely existed, but all knowledge of them is shrouded in mystery.
Most people south of the Wall believed the White Walkers to be a myth—something people told their children to get them to behave, but as we now know, they are real. The Wall was instrumental in keeping them at bay, but now that the wall is no longer a hindrance, the White Walkers will invade Westeros.
Not much is clear about what motivates the White Walkers in their quest to the South, though their purposes may be motivated by revenge. They can raise the dead to serve in their armies and are incredibly powerful, possibly undead, creatures led by the Night King.
Krakens are the sigil of House Greyjoy, which is appropriate given their nature. As far as anyone knows in Westeros, Krakens went extinct long ago, but they are commonly sighted by sailors who swear they still exist. There isn't much known about what they look like, but it is believed they are giant squidlike creatures.
Like mermaids (yes, they have them as well), they were once very common throughout the waters around Westeros, but have since disappeared. There isn't much known as to how this might have happened and nobody alive has likely seen one, which is one of the reasons they are considered legendary creatures.
Sea Dragons were once common throughout the waters of the known world, but have since made their way into legend as none have been seen for some time. It is said that the Grey King, who was the first to rule the Iron Islands slew Nagga, the first and mightiest of all the Sea Dragons.
Sea Dragons are similar to terrestrial dragons, though they do not have any wings and live primarily in the ocean. It has been thousands of years since the last Sea Dragon has been seen, but many in the Iron Islands still believe they roam the deepest depths of the oceans.
An Ice Dragon differs from a normal dragon in that it has a powerful freezing breath instead of fire. There is much online debate as to whether Viserion became an Ice Dragon when he died and was raised by the Night King, or if he became a Wight Dragon similar to other animals risen from the dead.
Regardless of Viserion's true disposition, the legendary Ice Dragons are said to be made of living ice. They can shoot off a freezing breath and are said to roam the Shivering Sea and the White Waste. It is also said that Ice Dragons are larger than the dragons of Valyria.
Dragons were long thought extinct before the events of the series gave birth to three under the watchful and caring eye of Daenerys. The Dragons in GoT are presented much like they are throughout fiction. They are incredibly large, armored, intelligent animals capable of flight, and they can breathe fire.
Many believe them to be more intelligent than men and they are also thought to have a strong connection to magic. Because they breathe fire and Ice Dragons (or White Dragons) are vulnerable to it, they take the top spot on the list as the most powerful legendary creatures in Westeros.