Any living organism that feeds on organic matter. Usually, they will have some sort of nervous system as well as specialized sensory organs. Dogs were probably the first animal that was domesticated around 15,000 BCE.
Throughout history, humans have been associated with animals, depending upon them for food, clothing, fertilizer, energy, etc. They have also been fascinated by animals, using sympathetic magic to acquire the strength of the tiger, the wisdom of the owl, the endurance of the camel, etc. Shamans would mentally travel to other worlds and communicate with animal spirits and guides.