Help: Alien/human assimilating creature

The Thing...

The Daleks from DW came to mind but they want to 'exterminate' not assimilate. Let me keep thinking...

I can't recall the title of the movie, the point is there were these aliens looking like manta rays, sticking to humans thus controlling them. I'll try to look up the title, but if someone knows what is it, don't hold back :)

Invasion of the Body Snatchers

There are weird slug-like aliens that inhabit the human host and control it directly in both the old Animorph children's book series as well as the more recent Stephenie Meyer book The Host.

Mara.

Zergs, maybe? Anyway, Tv Tropes is your friend

The Xenomorph? Absorbs traits of it's host. Best I've got.

Dudes from the movie Invasion of the body snatchers or from Puppet Masters or from The Faculty

Sil from Species?

Venom, from Spider-man comics. He's a symbiotic alien who needs a host.

Republicans! cue cliche comedy jingle

The ongoing series The Strain.
There're little worm-like things that once inside someone, procreate absurdly and change the person.

Those parasite worms from Futurama's 'Parasites Lost'?

Well if you want The Thing theme but need something less graphic, I advise you to read the reverse-engineered story here:
http://clarkesworldmagazine.com/watts.01.10/
Replace . character with _ at the last segment of the address.

I'd say the best for your purposes would be THE VENOM from Spider Man comics. People know about it from the movies, comics, and anime. Not too graphic and fits the bill.

The t-1000 from terminator 2!

Katamari from the PS games? Not an horror think but still assimilating everything.

The Human/Alien hybrid from Alien 4. Maybe it doesn't quite fit your requirement but still might be worth a shot.

The movie Society comes close.