STRINGS.CODEX.CRITTERSTATUS.DOMESTICATION.BODY.CONTAINER1
All critters are wild when first encountered, with the exception of babies hatched from eggs laid by domesticated adults—those will be born tame.
During the domestication process, the critter becomes less self-reliant and develops a higher baseline of expectations regarding its environment and care. Its metabolism accelerates, resulting in an increased level of required calories.
Critters can be domesticated by Duplicants with the Critter Ranching skill at the Grooming Station, and get excited when it's their turn to be fussed over.