Chao first made their appearance in Sonic Adventure in 1999 (December 23, 1998 for Japan) as a launch game for the Sega Dreamcast. Chao (pronounced "chow") were given a canonical history within the existing Sonic universe. Chao were seen to have existed since an ancient time, protected by a creature named Chaos; who was also the original guardian of the Master Emerald.
The player has been able to raise Chao for themselves in all the Sonic Adventure games, as well as a few other Sonic titles. The Chao are an evolution of the 'A-Life' system that Sonic Team originally created in the Sega Saturn Game "NiGHTS into Dreams..." using small creatures called 'Nightopians'. The Nightopians would change their behaviour depending on their treatment.
You can raise Chao in-game in special environments called Chao Gardens; special protected enemy-free sanctuaries designed for the safety and well-being of the defenceless Chao.
Chao hatch from eggs and grow through infancy into maturity. However, the way you care for your Chao will determine its physical characteristics, behaviour, personality, and way of life.
These Chao create a very unique ecosystem and learn and breed according to what you do. You must help your Chao to live by providing it with the necessary things such as food and attention.
Chao have become a popular character within the Sonic series, both from their appearances in the Chao Gardens and noteworthy cameos. This was greatly helped by the Sonic Adventure series being ported from the Dreamcast to the GameCube, PC, PS3, and Xbox 360 - opening the doors for a very large audience of people raising Chao, and ideally Chao Island will give them all the help they need.