It is my understanding that these things can live for hundreds of years if not killed, and with that in mind I believe they were once tamed and used by the people of Pulse. Although it would be some impressive feat, considering most people would die when they stomp their feet.

As for the behemoth thing - lions are known to hunt larger prey on occasion as a group, as are hyena and wolves. However, who's to say that they aren't protecting their territory from a behemoth which has wandered in? That's entirely reasonable.