The position and treatment of animals in Buddhism is important for the light it sheds on Buddhists' perception of their own relation to the natural world, on Buddhist humanitarian concerns in general, and on the relationship between Buddhist theory and Buddhist practice.
Etymology
In the Pali language, the translation is Tira-acchanā. Tira means against and Acchanā means a being that can move. Hence, the full meaning is a being that moves horizontally unlike the humans, Deva and Brahmā. Their world is called Tiracchana-yoni where yoni means "body".
In Buddhist doctrine
Animals have always been regarded in Buddhist thought as sentient beings. The doctrine of rebirth held that any human could be reborn as animal, and any animal could be reborn as a human. An animal might be a reborn dead relative, and anybody who looked far enough back through their series of lives might come to believe every animal to be a distant relative. The Buddha expounded that sentient beings currently living in the animal realm have been our mothers, brothers, sisters, fathers, children, friends in past rebirths. Therefore, one could not make a hard distinction between moral rules applicable to animals and those applicable to humans; ultimately humans and animals were part of a single family, they are all interconnected.
In cosmological terms, the animals were believed to inhabit a distinct "world", separated from humans not by space but by state of mind. This world was called Tiryagyoni in Sanskrit, Tiracchānayoni in Pāli. Rebirth as an animal was considered to be one of the unhappy rebirths, usually involving more than human suffering. Buddhist commentarial texts depict many sufferings associated with the animal world; even where human beings are not present, they are attacked and eaten by other animals or live in fear of it, they endure extreme changes of environment throughout the year, and they have no security of habitation. Those that live among humans are often slaughtered for their bodies, or taken and forced to work with many beatings until they are slaughtered at the end of their lives. On top of this, they suffer from ignorance, not knowing or understanding with any clarity what is happening to them and unable to do much about it, acting primarily on instinct.
The Chinese scholar Zhiyi taught the principle of the Mutual Possession of the Ten Worlds. This meant that all living beings have buddha-nature 'in their present form'. In the 'Devadatta chapter of the Lotus Sutra the Dragon King's Daughter attains Buddhahood in her present form, thus opening the way for both women and animals to attain Buddhahood.
In the Jatakas
The Jātaka stories which tell of past lives of the Buddha in folktale fashion, frequently involve animals as peripheral or main characters, and it is not uncommon for the Bodhisattva (the past-life Buddha) to appear as an animal as well. The stories sometimes involve animals alone, and sometimes involve conflicts between humans and animals; in the latter cases, the animals often exhibit characteristics of kindness and generosity that are absent in the humans.
Also recorded in the Jatakas is how, in a past life as King Shibi, Shakyamuni sacrificed himself to save a dove from a hawk. Recorded in the Golden Light Sutra, is how Shakyamuni in a past life, as Prince Sattva, came across a starving tigress and her cubs, he fed himself to them so that they would survive.
Behaviour regarding animals
The first of the five precepts bans the taking of life. The interpretation is that it applies to all sentient beings, which includes those in the animal realm in its broadest sense, i.e., not just mammals, but all animal taxa including insects and other invertebrates. From the beginnings of Buddhism, there were regulations intended to prevent the harming of sentient beings in the animal realm for various reasons.
The Buddha taught that from infinite rebirths, all animals have been our past relatives, sisters, mothers, brothers, fathers and children. Therefore, in Mahayana Buddhism, it is against the first precept to harm, kill or eat sentient beings as it is the same as harming, killing or eating the flesh of our own child or mother. Monks were forbidden from intentionally killing an animal, or drinking water with living creatures (such as larvae) in it.