Because in ancient times, animals were domestic things, people generally kept only six kinds of animals at home, that is, only cattle, sheep, pigs, horses, dogs and chickens, so six animals refer to these six animals, and it is very difficult to keep other animals at home, so six animals do not refer to other animals, but only these six animals.

In primitive society, people hunted for a living. They often go to the forest to hunt some animals and then use them as food. However, with the development of human beings, people find it very troublesome to hunt every day, so they think of a good way: domesticate these animals, and then if they want to eat, they can catch them directly without going hunting in the forest, so people gradually domesticate them.

When they said livestock, they also meant these livestock. They looked for many animals at that time and found that some animals were very fierce and difficult to domesticate, so they found that these six animals were relatively easy to domesticate, that is, they could become their own domestic animals, so the six animals at that time also referred to these six animals.

Moreover, these six kinds of animals have many benefits to people. Cattle and horses can help people transport, dogs can look after the house, pigs can eat, sheep can eat, hens can lay eggs, on the one hand, they can eat, on the other hand, they can hatch chickens. In the long run, there will be more and more chickens at home and roosters will crow. Mainly responsible for waking people up every day, and cocks and hens can eat.

In ancient times, the six animals people referred to were actually related to the environment of their family life. Generally speaking, they only keep these six kinds of animals at home, and other animals are not domesticated and naturally live in the wild. So in the minds of ancient people, there are only six kinds of animals, and other animals do not belong to six kinds of animals.