In the earliest days of the Third Era, when many of the Anu lived among humans as Gods, they kept humans as pets.
Rather than interact with the workmen themselves, they set up their favourite humans to rule over the others. What made a human a favourite was rather random: they might be pretty, or skillful at something of value, or possess a trait that entertained the Gods.
Generally, pets were treated well and kept close to the Gods as their servants or given responsibility over others.