Found this on Gfaqs. Seems pretty interesting.
To make Rock Eater appear, you have to kill 3 "Slavunell" creatures. The game gives you the hint that the Slavunell creatures aren't too friendly with the puny Mandragora creatures (from Final Fantasy XI) and suggests that you let the two creatures fight each other first and then go in for the kill yourself.
As they were at the game's E3 2004 debut, the similarities to Final Fantasy XI are clear here, even beyond the presence of the Mandragora beasts. Compared to most traditional Final Fantasy games, FFXII offers a surprising amount of freedom, giving you vast environments to explore and full camera control as you move about. There's no switch to a battle mode in this game -- battles are intermixed with dungeon crawling, making for a seamless experience.
he game also includes an interesting dynamic involving enemies. As the Fone Coast hint suggested, you're not the only one fighting the creatures of the FFXII world. They also fight amongst themselves, and you can see this as you move around (and take advantage of it in the Fone Coast demo). Just like in FFXI, once you've engaged an enemy, other creatures will take notice of this and will come in for attack, adding a new dimension to the closed battle systems from previous FF games.