An Eternal–Deviant warlord from the moon Titan, Thanos is regarded as one of the most powerful beings in the Marvel Universe. He has clashed with many heroes including the Avengers, the Guardians of the Galaxy, the Fantastic Four, the Eternals, and the X-Men.
Thanos is an Eternal, yes, but he's also a Deviant.
From his birth on Saturn's moon of Titan, Thanos was always an outsider. He carries a Deviants gene, which causes his gruesome and misshapen appearance, with purple, rugged skin. He was born among the last sons of Titan's original colonists: Mentor and Sui-San.
In the Eternals movie, Sersi said that the Eternals couldn't fight Thanos because they could only fight in conflicts that involved Deviants.
Thanos is not a robot.
In the comics, Thanos and Eros are born on the Saturnian moon Titan to two Eternal parents, A'lars and Sui-San. Eros is born without the Deviant mutation, but Thanos is born with Deviant Syndrome. They're biological brothers, but they look wildly different. So, in the original comics, Thanos is an Eternal, full stop.
Now, it's not explicitly said in Eternals nor in any other MCU movie if Thanos knew about The Emergence in Infinity War or not, but he most likely did.
Doctor Strange watches against threats from other dimensions, but the Eternals are part of our dimension. If it is revealed later that Doctor Strange knows of the Eternals, then the reason he didn't ask them for help in Avengers: Infinity War is that Marvel Studios wasn't ready to introduce the Eternals yet.
Eros, also known as Starfox, is a member of the genetically engineered Eternals. Created by Arishem the Judge in the World Forge, he was stationed on Titan, joining the royal family as a prince and becoming the adoptive brother of Thanos.
Arishem may have calculated a simple cost benefit analysis and determined that opposing Thanos given all of the aforementioned conditions simply wasn't worth taking the risk to himself.
That's why Thanos looks like a giant purple guy; Thanos' Deviant gene deformed his appearance. On the other hand, Eros looks like a regular human, just like other Eternals, because they belong to the same evolutionary path.
One Above All was one of the many victims of Thanos in his eternal quest to please Death. He was obliterated along his fellow Celestials by Thanos when he stabbed Black Bolt to death in order to use his destructive voice as weapon.
After all, It's because some fans thought that Galactus is a celestial, but no. Galactus is not a Celestial. Both are considered space gods threatening to destroy the earth. Both are giants with unimaginable power and have very similar wardrobes in the comics.
Peter Jason Quill was born in 1980 to Meredith Quill and Ego, making him a hybrid of human and Celestial. His conception was all part of a plot set up by his father to produce a second Celestial, whose power he could use to complete the Expansion, which involved assimilating millions of worlds into Ego's very being.
Where Eros serves as the superhero Starfox, he is opposed by his mad brother Thanos, much like the rest of the universe. He has the power to psychically control other people's emotions. He was a member of the Avengers and Dark Guardians.
Kang the Conqueror is human, but the answer is not as straightforward as we would have liked, as there are numerous versions of this time-traveling warlord, and some of them have been alien in origin. But the answer still stands, the main version of Kang from Earth-6311, Prime Kang, is human.
Superhuman strength: All Eternals have the capacity for superhuman physical strength. Starfox's physical strength is roughly average for an Eternal, and he can lift about 15 tons.
According to all the myths, Eros was beautiful, but also a permanent source of trouble for both the gods and the mortals. He appeared and threw his arrows, causing insane love and at the same time, suffering to people.
Hulk, Thor, and Thanos were not Starfox's level. Despite having a lot of raw power, he couldn't defeat his brother by himself. Assuming that Starfox's powers are anything like those of his comic counterpart, he would have fared much better against Thanos than most of the Avengers in both Infinity War and Endgame.
Kingo said Thor used to hang out with him when he was a kid. That means Thor knows about the existence of the Eternals.
Odin, therefore, also knew about the Eternals and, given Odin's demonstrated knowledge of MCU history, probably also knew about the Celestials as well.
THE FIRST ETERNAL AVENGER
Technically, Gilgamesh and Sersi were the first Eternals from Earth to become members of the Avengers. But the first Eternal Avenger was none other than Eros, AKA Starfox. Starfox came to Earth in AVENGERS (1963) #232, and he immediately asked to join the roster.
"Thanos' skin is almost impenetrable, we don't know whether Doctor Strange had the capability to do it," Russo said. "If he failed to cut it on time, Thanos would still able to do the snap. Doctor Strange realized this issue during his millions of test runs." Spoilers for Avengers: Endgame follow.
When Thanos snapped his fingers, he delayed the Emergence from happening as the population was cut in half. However, when the Avengers brought everybody back, they unwittingly started the countdown to the Emergence and the end of Earth as we know it.
In the comics, Eternals aren't allowed to conceive a child. But this was proven otherwise when Thanos was born from two Eternals. As a result, he becomes host to the Deviant Gene, which alters his appearance but still technically makes him an Eternal.