Trolls.
In most fantasy genres trolls are large, stupid creatures that live a savage lifestyle. In fact, there is a trend in these genres that all non-humans (trolls, goblins, ogres, orcs, gnolls, kobolds, etc) are stupid, savage minions. Warcraft has always gone against the grain on this - even relatively stupid races like Ogres and Kobolds have some feral cunning and magical ability, and more human-like races such as Trolls and Goblins can be as intelligent or even more intelligent then humans and elves.
Trolls in Warcraft has always fascinated me. Ever since their introduction in Warcraft II as a replacement for the Orcish spearthrower, I've loved the idea of the trolls. Vicious, fiercely territorial, xenophobic (as many great species tend to be) and possessing unusual skills with thrown weapons and the ability to regenerate from grievous wounds. My interest only deepened as the lore of the trolls expanded to describe the ancient empires of Zul'Aman and Zul'Gurub, the many MANY different species of trolls, and their strange symbiotic/parasytic connection with their Loa Gods.
As the lore has expanded, it's become more and more likely that Trolls, specifically Zandalari Trolls, were the original civilized race on Azeroth, possibly even pre-dating the Titans. While other races such as Gnolls, Kobolds and Tauren may have existed at this time, they were largely in the tribe-based hunter-gatherer stage of development, while Trolls were building their first Empire. The trolls may have worshiped the Old Gods, but it is much more likely that even then they worshiped the Loa, or Animal Gods of their current faith, since it is made clear in the War of the Ancients trilogy that the animal Gods are quite old. The trolls built a mighty empire, and conquered a large swath of Kalimdor.
The fall and eventual split of the troll Empire in to the Amani Troll (forest troll) Empire and the Gurubashi Troll (jungle troll) Empire was brought on by their conflicts with the Aqir, the progenitors of the Qiraji and Nerubians (and possibly the Silithids as well). It's unclear if this was the first split in the troll power structure, some lore suggests that the Drakkari Trolls (ice trolls) may have been exiled at an earlier time for their violent tendencies. Regardless, a small segment of the original Zandalari trolls remained, granted a position of respect and leadership over the other trollish people, and the remaining trolls split in to their respective empires and began to genetically diverge.
Of all of the creatures of Azeroth, Trolls show the greatest propensity for genetic divergence. Trolls rapidly evolve in to separate subspecies to best suite their surroundings. Amani trolls evolved in to Forest Trolls, Gurubashi in to Jungle Trolls, Drakkari in to Ice Trolls, Farraki (who may have been isolated on Kalimdor during the Sundering) in to Sand Trolls, and so on and so forth. It has further been hinted that the Night Elves were originally a group of Trolls who settled on the banks of the Well of Eternity and were enhanced by it's power. An initial transformation in to the theoretical Dark Trolls may have occurred as those on the Well's shore became nocturnal, and then the power of the well may have finalized the process in those that remained, stripping them of their trollish regeneration but evolving them in to the graceful, civilized, powerful beings they were to become.
If that is the case, it would certainly explain the Night Elves own genetic mutability, their split in to the races of the High Elves (and later the Blood Elves and Felblood Elves) and Naga due to environmental changes. It is also notable that Goblins have many trollish attributes and a high level of intelligence, and may have been the result of Trolls venturing underground and evolving in to a short, inventive race suitable for the environment beneath the world. Certainly Goblins predate the Sundering as they served Neltharion the Earthwarder (later to be known as Deathwing the Destroyer) during his madness.
If the above conjecture is true, this means that all of the native civilized races at the time of the Second Invasion of the Burning Legion were of trollish decent. Trolls, Goblins, High Elves and Night Elves would all be Trollish. Dwarves, Gnomes and Humans are now all confirmed to be the descendants of Titan seed races corrupted with the curse of the flesh. Orcs, obviously, were descended from the First Horde that came through the Dark Portal from Draenor. At the time of the Second Invasion Tauren were still nomadic hunter-gatherers and settled in Thunder Bluff after this - it's arguable whether they were civilized or not, and I would not be surprised if we one day learned more about the origins of the animal hybrid races such as Tauren, Gnolls, Quillboar, etc.
There's a lot more to write about Trolls, their beliefs, their culture, and how their actions have shaped the world and the lore, but this is a good starting point to understanding how interesting and complicated their history (and biology) truly is.
Aedis
38 minutes ago