:x originated with me because I am a trendsetter, naturally.
When I was little and in elementary school, I used to think I was clever and funny when I put my fingers in an X shape over my mouth when there was something I didn't want to say or when I was in an awkward situation. It caught on with some of my friends, and we all started doing it, and then stopped after a while, as fickle kids do.
A couple years ago, I remembered this practice, but instead of doing it in real life (because that would make me even creepier than I already am) I translated it into an emoticon, most accurately represented as :x.
Almost everyone I talked to at that point then adopted the smilie because I used it so avidly in conversation. Most relevant :x early adopters were Genius and Fuzzy, who then did quite a bit to propagate the inside joke.
I'm curious about this 
It's known as a dargon, created by Madrigallia, who was a member here for a very short amount of time a couple years ago. Real-life friend of GF's.