Buddhists are expected to show metta to all creatures:
Buddhism is a religion of love, understanding and compassion and is committed towards the ideal of non-violence. As such, it also attaches great importance to wildlife and the protection of the environment on which every being in this world depends for survival.Buddhist Declaration on Nature, Assisi 1986
Buddhists believe in ahimsa - doing no harm and being non-violent. It means avoiding any deliberate harming of any living thing and having a desire to bring about the greatest good.
There are stories found in the Jakata Tales, which describe the Buddha's previous lives. Some of these stories depict the Buddha as an animal in a previous lifetime during which he perfects one or another virtue, such as courage, or compassion or patience.
The first of the Five Precepts of Buddhism is:
I undertake the precept to refrain from taking life.
Some Buddhists interpret this to mean that Buddhists should not kill any living thing, including animals for food. Others believe this refers to human, not animal life and therefore killing animals for food is acceptable. However, no living thing should be killed unnecessarily.
Buddhists must choose work which enables them to follow Buddhist principles. Therefore they prefer not to take part in the trading or slaughter of animals, in fishing, or in the breeding of animals for slaughter.
Humans live a continuous series of lives on Earth, some of these possibly in the form of animals. This belief is known as rebirth. As a result, Buddhists believe that human beings are all interdependent. This means that in their previous lives, those animals living around us might have been a member of our own families. Tibetans say that all living creatures have at one time or another been our mother. Therefore, people should respect all animals. This is why some Buddhists choose to live a vegetarian lifestyle. For example, Chinese Buddhist monks and nuns are very strictly vegetarian. However, other Buddhist countries do not expect monks and nuns to refrain from eating meat or fish.
…in the continuous birth and rebirth of sentient beings (not only on this planet but in the universe as a whole) each being is related to us ourselves, just as our parents are related to us in this life.Buddhist Declaration on Nature, Assisi 1986
Buddhists have respect for all life. Life is involved in a continual cycle of birth and death known as samsara and freedom from this cycle comes through enlightenment. Buddhist believe in karma or 'intentional action'. Taking the life of an animal unnecessarily is a bad action and this will have an effect on the quality of the next life. The outcome might be rebirth in animal form where one experiences the same kind of mistreatment.
Many Buddhists believe that humans have abilities which animals do not have:
Generally speaking, human beings are superior to animals. We are equipped with the ability to judge between right and wrong and to think in terms of the future…Dalai Lama
This kind of superiority does not mean that animals cannot feel and suffer as humans do. So the lives of humans and animals should be treated with equal respect. In some forms of Mahayana Buddhism, animals possess 'Buddha nature', ie the ability to become an enlightened being.