The shulassakar crop up in Eberron materials for both 3.5 and 4e, and they have a brief uncited mention in the Wikipedia article on yuan-ti. However, the rainbow serpent folk never seem to get much attention: that Dragonshards article I link above has the most information in one place that I've ever found.
Where/when did they first appear? Were they invented specifically for Eberron in the early 2000s, or is there some previous iteration of the "couatl people" concept on which Eberron is drawing? What are the best (comprehensive and coherent for both crunch and fluff) resources on them?
(And if it's not answered as part of the above--is there any art for these colourful guys?)