Some civilizations do not have access to the Livestock Pen, but have replacement buildings instead. Native American civilizations have the Farm instead of Livestock Pen which combines the functions of a Mill and a Livestock Pen, while the Chinese have the Village instead of Livestock Pen which combines the functions of a House and a Livestock Pen.
Two civilizations do not have any access to the fattening capability because their Villagers cannot gather food from Herdables. The Indians have the Sacred Field instead of Livestock Pen that increases the experience generation rate of their livestock; the Japanese may use Shrines to generate resources over time.
Livestock Pens effectively cut the fattening time of most livestock down from 30 minutes to around 5-6 minutes. However, they are less common than gathering food directly at the Mill, especially in Supremacy. This is due to the more complicated mechanics of gathering from herdables, such as the upfront cost of invested food that cannot be used to train units.
If there is a 5-6 minute period for livestock to be fattened at the pen, such as during a Treaty match where military units are not required, livestock can be used as a worthwhile investment to multiply one's food. It is easy to adjust the amount of food and settler time invested by training more livestock units. The Fulling Mills card indirectly relates to this building by making it 4× faster to gather from herdables, saving villager time, reducing waste to spoilage, and freeing them to do other tasks.
Argentina: Gauchos and Granaderos can build Livestock Pens, train Cows, and gather food from livestock.
Brazil: Jaguncos Ships 10 Cows and 10 Jaguncos; Cow train limit +10; Jaguncos can build Livestock Pens, train Cows, and gather from livestock.
Mexico: Vaqueros (Comancheros) can build Livestock Pens, train Cows, and gather food from livestock.
Peru: Morochucos Ships 10 Cows and 10 Morochucos (Comancheros) can build Livestock Pens, train Cows, and gather from livestock.
United States: Cowboys Ships 10 Cows and 10 Cowboys (Comancheros) can build Livestock Pens, train Cows, and gather from livestock.
Keeping animals was important to the founding of the New World, especially as areas became settled and hunting became less abundant. Animals were used for food, but they had other uses as well. Fat from animals was used to make soap, the skins were used for clothing, and tallow was used in candles. Tallow was also used to keep leather supple and water-resistant.