Tahtorak

Tahtorak were massive 40 foot dragon Rahi that could speak Matoran. They are native to both the island of Keetongu's species and to Zakaz. Tahtorak were used as steeds for Keetongu's species and still are for the Skakdi.

An Order of Mata Nui member was told by a dying Tahtorak that its kind have had enough of the Skakdi's warring and destroying of Zakaz, and plan to wipe them out.

The Tahtorak of Metru Nui
A Tahtorak was dropped onto Metru Nui by Brutaka via his Kanohi Olmak for fun some time in the past. It went on a rampage in Ga-Metru before being brought down and locked in the Archives. Through unknown circumstances it somehow became buried deep in the Fikou Web, where it lay asleep until it was accidentally awakened by a number of Lava Eels the Toa Metru agitated during their conflict with Krahka. Later, it tore its way to the surface of Ta-Metru where it proceeded to lay waste to the city, all the while asking for an "answer" to a question no one could understand, although Rahaga Pouks theorized that it asked how it came to Metru Nui, so far away from its home.

The Toa Metru defeated Tahtorak by weakening the ground until it fell through. Much later, that same Tahtorak reappeared when the Visorak overtook the city. Krahka managed to get it to ally itself with her, the Toa Hordika, and the Rahaga. It fought a brave battle against the Zivon, which resulted in Krahka, Zivon, and the Tahtorak to be thrown into the Zone of Darkness.

1,001 years later, he and Krahka traveled throughout the Field of Shadows again, only to find themselves exiting through a vortex created by Brutaka inside the Piraka Stronghold. Krahka shapeshifted into a flying Rahi and flew away from Voya Nui, but the Tahtorak remained behind.

After the Toa Inika retrieved the Kanohi Ignika and Vezon and the Kardas Dragon were separated, Tahtorak ran into Kardas and they began to battle one another. In the end, Tahtorak won. Annoyed, Kardas fled the island towards Mata Nui. Axonn knew Tahtorak would be a threat to all who lived on Voya Nui, so he summoned Botar to transport Tahtorak elsewhere. For fun, Botar teleported him into the very center of Xia. He went on a rampage in the city there, fighting the Kanohi Dragon until the Toa Hagah met both of them. The Tahtorak was brought down by Bomonga using his Mask of Growth. Pouks then used Stone bonds to contain the Rahi.

Trivia

 * Tahtorak was a winning design in the Rahi Beasts contest.
 * Tahtorak was on the cover of the Rahi guide.
 * "The Tahtorak of Metru Nui" isn't an offical name, just a nickname.