'pet' originates from Middle English (16th century) and likely from a Celtic source such as Irish or Scottish Gaelic 'peata' meaning 'tame animal' or 'pet'; by the 1500s it was used to mean a favourite person or animal.

'pet' appeared in Middle English as 'pet' meaning 'darling, tame animal'; over time the sense narrowed and by the 17th century it commonly referred specifically to a domesticated animal kept for pleasure, giving the modern word 'pet'.

Initially it meant 'a favorite or tame thing/person' (and could apply to people); over time it evolved into the more specific modern meaning 'an animal kept for companionship or pleasure'.