Ruminants are called some mammals, because they are ruminating.
This category includes many of the well-known large mammals, like cattle, goats, sheep, deer and antelopes.
They use fermentation and digest food in two stages. Initially they chew and swallow in the normal way, and then the semi-processed food (partially digested) known as cud returns to the mouth for more chewing. In this way they are able to acquire nutrients from plant-based food by fermenting it in a specialized stomach prior to digestion, principally through microbial actions.