Thanos, the ultimate MCU bad guy, is actually a Deviant too. He's by far the most famous of the lot, and though that doesn't necessarily mean he'll appear in The Eternals movie, his presence goes someway in reiterating just how influential it may prove to be upon release.
The Mad Titan isn't the only MCU villain related to the group though - the Skrulls, who've already made their MCU bow in Captain Marvel, are also actually a species of Deviants, albeit without the individual mutations of their close relatives.
So, there you have it. If you've ever wondered why Thanos has a similar indented chin to that of the Skrulls, it's because they're both Deviants. The more you know!