Prince Samatv unleashed far more chaos than he planned.
It wasn't his original intention. But the initial beasts he released broke the chains of other beasts and one of these beasts happened to be recently caught—meaning, it wasn't domesticated enough to have its mind nearly broken down to its basic insticts.
So, this sane beast turned into a commander and used the more powerful beasts to lay down a trap.
Even though they weren't as clever as a normal humanoid race, any creature that reached rank 9 or even rank 8 for that matter would not be devoid of basic intelligence.
Their high rank ensured long lives. So, even if a beast was really dumb at learning things, it'd eventually grow very wise.
So, the challenge that confronted the guardians of the laboratory wasn't a horde of mindless beasts as they initially judged.
It was an army of beasts led by a mastermind.