Hmm, actually, I've always assumed it's a Tarek, too. If it's supposed to be a mul, the race underwent quite a bit of redesign until they released the box...That's a mul and some giths on that cover.^^
In any case, there's no reference to tareks in my old 2e DS stuff (old box and first MC) that I'm aware of.
Tareks are mentioned in the Prism Pentad novels. I'm not sure if they ever made it into an official supplement or Dragon article, though. They were definitely available as a player race in my 2e Dark Sun campaign (along with pretty much every other remotely humanoid-shaped beings).