When I first saw these guys, I immediately thought "Okay that's weird but not totally unheard-of."
Reason is, my sister used to own a Green iguana. Ralphie, like all iguanas, had a
parietal eye on the top of her head, between her actual eyes and back a scootch. it looked like a slightly different scale. While in most critters with a parietal eye it's just photoreceptive, in
Tuataras it's more developed. Quoth wikipedia: "It has its own lens, cornea, retina with rod-like structures, and degenerated nerve connection to the brain, suggesting it evolved from a real eye."
Maybe Azeroth's basilisks have a third eye much like a Tuatara or Iguana (on whom it's fairly hard to see unless you know what you're looking for), and in the Outland critters it's still an actual eye? While it's not in the same place on their heads, there may be a crazy WoW biology reason for that. Whether it's 'Azerothian basilisks have a parietal eye that evolved form an actual eye' or 'Outland basilisks are Azerothian critters whose parietal eye evolved back into a third eye because WOOO TWISTING NETHER' or what, I can't really say!
Either way, a lizard with three eyes isn't impossible. WE HAVE SOME HERE XD it's just not an eyeball-eye as such.
Also, it's interesting to note that dragons, proto-drakes and basilisks are all six-limbed (yes, protos have six limbs, their foremost ones are just ittybitty)...and that Proto-drakes use their wings as an extra set of walky-legs/balancin' arms when they're on the ground. And they still have the 'lisk underbite (and so do Dragons though it's less pronounced). So you can see where the Titans went *pokity pokity*.
IT'S REALLY COOL XD I like when IRL biology and game biology make sense in relation to one another.
(I fully blame OMFG.FM podcast archives for getting me to think of this, specifically the Abstract Polygon segments of Council of Six. Someday I will figure out how to thank Strixus for being the second person to let me know I'm not alone in going gosh-wow nerdity over little details. My awesome girlfriend is the first one who did, and she is the most instrumental in encouraging me to do this nerdery; we have long discussions about Azerothian cuisine and B'elf desserts, so. XD)