Hekatons were absolutely massive beasts native to the planet Stygia. Those people who came to settle the world believed them to be but only a myth, but no known individual actually witnessed it; however, how this fits with their basic knowledge of its existence is unknown, whether they read about it somehow in ancient texts from some long-lost race or some random individuals heard or saw it and their reports were simply taken as folklore. It also does not explain why the legends of the beast's appearance are spot-on, as the Stygian people had an amusement park that utilized Hekaton themed robots called Mechatons that looked just like properly-scaled adult Hekatons.
Eventually, Grayson Hunt and Ishi Sato came across the subterranean nest of one of these creatures, and were forced to destroy her eggs in order to escape from a cavern collapse. This proved itself to be a very bad idea, as Hekatons are apparently extremely protective over their clutches and presumably their young, and this single act caused the Hekaton mother to pursue the two throughout their journey across the planet, causing mass destruction in her rampage.
Hekatons are extremely resilient, and the one faced by Grayson and the other survivors refused to die no matter what they threw at it until they detonated the DNA Bomb, which eradicated all life on the planet that possessed DNA. It is also noted by Grayson that the eggs they had smashed smelled horrible, which either indicates that they're naturally this way or that they were simply all rotten to begin with and that the mother will still guard over dead eggs. Several massive skeletons were also encountered by the team on their journey, which can be presumed to have been Hekatons, as well. If so, this may indicate that they were once more prolific than they were at the end of the planet's evolutionary history.