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.
A Titan among Men
Physically, Thanos stands head and shoulders above the average human or humanoid, with powerful muscles, a broad face, and a purple-hued skintone. He often wears a protective suit of armor and his strength is such that he has been physically able to battle—and defeat—the likes of the Hulk.
In the comics, Thanos is an Eternal, and one from a very important bloodline: His father, Al'ars, was the son of Kronos, who ruled the Eternals of Earth following a civil war that divided the original colony.
Thanos is not a robot.
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.
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.
Most Eternals can lift about thirty tons, with the stronger ones like Gilgamesh, Ikaris, and Thanos being much stronger. Eros is actually weaker than the average Eternal, but he's still much stronger than the vast majority of humans.
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.
The abilities of the Eternals - their strength, speed, power, intelligence, experience - outmatch the Avengers' at almost every turn.
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.
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.
The Eternals were instructed not to interfere with any human conflict unless Deviants are involved.
Shortly after Starfox joined the team, the first encounter between the Avengers and the Eternals unfolded in AVENGERS (1963) #246-248.
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.
Thanos didn't lift Mjolnir, Captain America did
With Mjolnir's worthiness enchantment resulted in Thor being able to lift the hammer only if he is considered to be worthy, but likewise, the enchantment is not discriminatory in nature, and anyone who is deemed worthy is able to lift the hammer.
When it comes down to it, Thanos knows Stark as the man who thwarted his efforts to take over Earth via Loki in 2012's The Avengers. "Which is why he's aware of Stark from the original Battle of New York as the person who undid the plan," Joe Russo said, giving extra meaning to Thanos having respect for Iron Man.
He instead exists in a realm beyond space and time, he's immortal, and he can manipulate matter, energy, magic, space, and time however he sees fit. Every universe and living thing within the multiverse, therefore, is part of Eternity.
It would be interesting to see who would win a battle between them if only skills are concerned. But we all know ultimately Thor will emerge victorious given his strength, endurance and magical powers.
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.
Thanos is a mutant member of the race of superhumans known as the Titanian Eternals. The character possesses abilities common to the Eternals, but amplified to a higher degree through a combination of his mutant–Eternal heritage, bionic amplification, mysticism, and power bestowed by the abstract entity, Death.
While Thanos is an Eternal and has the standard powers of super-strength, agility, regeneration, immortality, and more, Adam Warlock takes the upper hand. He can be killed, but he resurrects to be stronger every time.
With a simple thought, the Beyonder can displace an entire planet, change the rules of physics, and even destroy the entire universe. That's why the Beyonder was presented in Marvel Comics as the most powerful being to ever exist.
Starfox was not in the same class as Hulk, Thor, or Thanos. So despite having plenty of raw power, it was never enough for him to overpower his brother on his own.