Zaetar

Zaetar was a keeper of the grove, the eldest son of Cenarius. He was seduced by the elemental-princess of the earth Theradras, and taken to Maraudon. The product of their union were five tribes of centaur (Magram, Kolkar, Galak, Gelkis and Maraudine), twisted mockeries of the beauty of the dryads and keepers of the grove. According to the story, the first act of the jealous and evil centaur was to kill their father. Fearing Cenarius' wrath upon her children, Theradras spirited Zaetar's remains, and his soul, away to be with her in Maraudon.

As he is the eldest son of Cenarius he is likely the one known as "the keeper of the grove" in the night elven legend that speaks of three oldest offspring of Cenarius, his sister "the dryad" and his bastard brother, "the centaur" (the first of those races). Keeper Remulos is one of his younger brothers not mentioned in the legend. Zaetar's nephew, Celebras, the son of Remulos, attempted to rescue Zaetar's spirit from Maraudon, but became corrupted himself by Theradras' magic, and still resides there with his dryads.

As he only fathered five tribes according to World of Warcraft and Lands of Mystery, he is not likely the progenitor of the Krenka, the Marauders, the Stonetalon tribe, or Eastern Kingdom centaur tribes. Instead those centaur were likely were the sons of Zaeter's bastardized brother mentioned in night elven legend, "the centaur".

You will encounter Zaetar's Spirit in World of Warcraft, after defeating Princess Theradras.

Stormrage
In the novel Stormrage, Broll Bearmantle is lead to Zaetar while in the Emerald Dream. Zaetar was leading a group in battling the Emerald Nightmare. He is described as having a greener tint than Remulos, and a faintly longer face that bore in it a constant sadness.

Speculation
Alternatively, some believe that the two legends (this one and the night elf legend) are one and the same, in which the son called "the centaur" may in fact be Zaetar. This is because they share many things, such as their fall from grace due to jealousy. This would mean that "the centaur"'s children were the same as Zaetar's. Those also believe that the Krenka, Marauder, Stonetalon, and Eastern Kingdom centaur may have simply broken away from the five main tribes in the countless centuries since their creation, as the five tribes remaining unbroken for well over ten thousand years may seem unrealistic.

However, this would ignore most of the traits, and history attributed to "the centaur". Zaetar appears to neither be a bastard, nor does he have the centaur's personality traits. He does not have the same hatred of his siblings as "the centaur". Zaetar was not cursed by his father either (stripped of his beauty to appear as current centaurs), nor did Zaetar swear a blood feud on all creatures of the world. These discrepancies could be explained as exaggerations added as the story was retold over the years.