AMNION

Amnion is an adavnced space-faring vaguely humanoid species which looks like " "a humanoid sea-anemone with too many arms." They differ greatly in appearance from one another, with differing numbers of limbs, eyes, and other organs, as well as  in size and shape. While being physically very different, they share a hive mind with the major instinctive drive of the expansion and propagation of the Amnion species, leading them to assmiliate other species by turning them into members of their own.

They possess  mutagenic technology that gives them the ability to rewrite the DNA of a non-Amnioni to turn him into an at least partially Amnion. Basic life processes remain intact, but protein and DNA structure are radically different. The new mutants can breed with each other (though not with mutated members of other species) and display a significantly increased level of intelligence.

First contact was made betwwen them and humanity while humanity was already travelling interstellar journeys.
Source: Stephen R. Donaldson's The Gap Cycle