A large majority of the show's dialogue is improvised.
The project bypassed the traditional development, and was financed with private equity money. The creators were able to make the show exactly how they wanted, on a timetable that suited them. So, they did the first two episodes, and shopped them around different networks.
The animals all can speak clean English, whereas the humans seemingly speak in unintelligible grunts and moans.
All episodes' lead two characters/animals are voiced by the show creators Phil and Mike. Phil and Mike also appear in many of the live action sequences.
Nathan Fielder, from the Comedy Central show Nathan For You (2013), voices the character DJ Lab Rat from the first episode, Rats.