They take turns standing guard when it is time to sleep. The sentry awakens the others if it senses danger. If one becomes separated, it turns sleepless with fear.
(From Bulbapedia) Sentret is a small, furry Pokémon with a big round body, stubby arms, and two small feet. Its fur is light brown, and it has a tail with dark brown stripes that it uses to stand on to see if there is danger nearby. It has long dark brown ears, a white ring on its belly, and white paw pads. While it is capable of standing on its hind legs, it usually moves on all four. Sentret behaves very nervously and cautiously, apparently fearing for its life. It will not sleep alone, and usually sleeps while another of its kind watches for danger. When one sees a predator, it warns the rest in its pack, and they all flee. Sentret can most commonly be found in grasslands.
(From Bulbapedia) Despite its name, Sentret's behavior and markings are more similar to meerkats than ferrets. It also shares traits with tanuki, flying squirrels (more specifically, the Japanese giant flying squirrels), and sugar gliders.
(From Bulbapedia) Sentret is a combination of sentry and ferret.