Karnage1992 said:

i'm glad i'm not the only one who had that notion.

I was recently reading the book Fulgrim and then just looking at the Sslyth model I'm like "hey! it's one of the Laer!" lol

There's also the Enoulians from Dark Heresy's Creatures Anathema. The remains of a shattered alien civilisation who work as mercenaries but always against the Imperium and in recent times they are ever more often sighted side by side with the forces of Chaos…

There are a bunch of Chaos-worshipping xenos in the Gaunt's Ghosts novel Traitor General. I think it's weird that Games Workshop has never developed 'Chaos Xenos' as a troop type in the tabletop game…

I'm surprised nobody has mentioned Loxatl 

http://wh40k.lexicanum.com/wiki/Loxatl

Even if not as a playable race, they're a great start for minions

I was hoping to get a big dark eldar write up in the slaanesh book, same way necrons got one in the tzeentch book, possibly with rules for using them as player characters somehow. Then I found out rogue trader already had a whole dark eldar book, and kinda figured that would be a no.

Still, I'm holding out hope that after the 4 main books are done, maybe with chaos undived as well, we could get some more obscure source books, like dark mechanicus and especially chaos xenos.