The frequency in which animals are encountered in a role playing game depends a lot on the setting. In fantasy games, animals and monsters can be the most common foe encountered. In a modern or futuristic setting they can be little more than part of the background narrative. Even if characters don't encounter animals in anything other than a descriptive sense (you don't need game statistics to describe the rats scurrying underfoot whilst the PCs clamber into a modern sewer looking for cultists), some idea of how the game can describe the world can be useful.
The aim is to provide a wide range of example animal statistics from the real world, as well as some more common examples from horror, fantasy and science fiction. As a GM, you may never need the statistics for an elephant, but if you need something vaguely elephant sized, then elephant statistics can act as a good starting template.