 If I assume that Matthew is asking about which animal emits more greenhouse gases by itself, we should go for a herbivore. When herbivores digest the grass that they eat, they emit methane, which is a very powerful greenhouse gas. And the amount emitted will depend mainly on the size of the animal, but also on the type of the digestive system that they have. So in this case, ruminants have a very complex and efficient digestive system that tend to emit quite a lot of methane.