The small digital pet species that can be raised in Sonic Adventure is Chao.
Chao are small, fairy-like creatures introduced as a virtual-pet feature in Sonic Adventure. Players find eggs in Chao Gardens, hatch them, and care for them by feeding, holding, and giving them toys. Their appearance, abilities, and behavior change according to how they are raised, making them more personal than ordinary collectible characters.
The system connects the quiet Chao Gardens to the main action stages. Defeating enemies can release small animals that improve a Chao’s statistics, while rings can be spent on food and toys. Chao can then compete in races, turning the platforming levels into a source of resources for a separate raising simulation.
Sonic Adventure’s Dreamcast version also supported Chao Adventure through the console’s Visual Memory Unit. A Chao could be transferred to the memory card’s tiny screen-based game, where it gained skills and items before returning to the main game. Later entries, especially Sonic Adventure 2, expanded the feature with Hero and Dark Chao, but the species began with Sonic Adventure’s gardens.