Worst gang-like behavior of animals I every saw was, like, a decade ago at the Cape Point parking lot in South Africa.

This a relatively large lot (being at the round-turn point of a one-way route), with both personal vehicles and tour busses coming and going. There are also a lot of baboons. On the narrow-ish mountain road up and down, these already pester the vehicles every way they can, but once parked, it's when things get truly special, especially for the tour busses.

First, the baboon females parade past a bus, with infants on their back. The tourists are delighted by this, and rush out with their cameras. Then, the females retreat into the bushes, which further distracts the humans.

Then, the males, which have been hiding in the nearby trees, descend upon the bus, and absolutely ransack it. Everything that can be taken (and that's mostly bags and coats) gets dragged into the trees. Then, they triumphantly go through the loot, redistribute anything that's edible or otherwise interesting, and toss the rest.

It's scary how well these animals instinctively understand the way we behave...