Speaking of DS9, the Changelings. Their natural state is as an ocean of goop all globbing together.
More awesome in that Jophur without Master rings are another,altogether much nicer if somewhat indecisive, race entirely (Jophur are kind of supremacist dicks - very much like Daleks, that’s how I “heard” them in my head). And the Master rings are an outside imposition by their “progenitors”, so it’s a sad story overall - think “The Puppet Masters/Body Snatchers” but everyone’s alien.
There’s a weird case in Michael Flynn’s Eifelheim - the aliens are a bit strange (having a hierarchy that is genetically coded into them, and thus very hard to resist) but in many ways, the humans of 14th century Europe are more alien to the modern point of view than the real aliens are.
Iain M. Banks has several species that fall into this category (of course, he also has the very unlikely pan-human galactic agglomeration that is the Culture)
- The Dra’azon - incorporeal species that protect the “Planets of the Dead” for unknown reasons: http://en.wikipedia.org/wiki/Dra’Azon#Dra.27azon
- Dirigible Behemothaurs, and “megalithine” and “gigalithine lenticular entities”, kind of like slow, inscrutable, ancient intelligences that look a bit like a cross between a gigantic blimp and a whale.
He also has lots of species that are variations on themes you might see on Earth, of course.
The ants in Phase IV were even stranger.