The Escape from the Philadelphia Zoo was an attempt by Casey to escape The Beast.
Background[]
When the Beast closes in on her, demonstrating the ability to climb up walls, Casey uses Kevin's real name to call him to the light. The Beast is briefly tamed, and the actual Kevin (who has been restrained for a very long time by the other personalities, and has no memory of what he has done between then and now) is shown to be horrified by what he has done, and orders her to kill him with a shotgun before the Beast can return and defend itself - however, the Beast is alerted when Kevin's other personalities desperately try to defend themselves.
Battle[]
As the Beast reawakens, it engages in a climactic and violent showdown with Casey. Casey fights the Beast, shooting him several times with a shotgun, but it proves to be superhumanly tolerant to pain. At the climax of the fight, the Beast voices his maniacal plans to rid the world of the 'untouched' (As in, the people who he feels have never suffered pain or discomfort, unlike him). Casey is cornered in a cage, but the Beast bends the bars with its bare hands and is about to attack her when it notices the scars on her body from her previous abuse at the hands of her uncle - it rejoices in the fact that she is 'pure' (she has suffered).
Aftermath[]
Speculating that troubled people are exceptional, the Beast decides to spare Casey and flees, while Casey is eventually discovered and rescued by Kevin's co-workers at the Philadelphia Zoo. Recovering from the battle, Patricia, Dennis and Hedwig speculate in awe at the power of the Beast, marvelling at what it was shown to be capable of doing. Kevin voices his plans to change the world, and awaits the next feeding of 'the Beast' now under the man's newest and diabolical collective nom de guerre "The Horde".