In the Marvel comics, Thanos is of Eternal lineage but was born mutated, not unlike the Deviants.
Thanos’s lineage was explored in an explainer video on Marvel’s YouTube channel which reveals that Thanos’s father, A’Lars was an Eternal.
However, while the Eternals appear to be human, Thanos was born with purple skin and was mutated like the Deviants.
As a result, Thanos cannot be categorically described as a full Eternal or full Deviant as he treads the line between both groups.
Perhaps then, this is the reason why the Eternals didn’t intervene when Thanos attacked Earth as he was technically one of their own rather than an out-and-out Deviant.