Among several other gasses - methane is probably the most prevalent, followed by hydrogen sulfide, which gives off the characteristic rotten eggs odor.
An interesting question, thought provoking. Many a time have I also asked myself, "Why do animals gas breathed out by animals?" The questions depth and intellectual prowess never cease to amaze...
The waste gas given off by animals is methane gas (CH4). It is often produced as a product of the bacteria in the digestive system or gut of many animals, such as ruminate animals like cows.