But this is a diversion: The meaning of pet that we are interested in does not, sadly, come to us via this route. Instead, it is from the Scottish Gaelic peata, which meant “tame animal.” In Robert Pitcairn’s Ancient criminal trials in Scotland (1488–1624) we find the 1539 comment “…deliverit to Thomas Melvillis wiffe, in Falkland for keeping of certane pettis and nurising of the samyn.”
By the end of the 17th century and beginning of the 18th, the word had come to be used more specifically to refer to a young lamb. In 1755, Samuel Johnson’s entry for pet in his Dictionary of the English Language was, “A lamb taken into the house, and brought up by hand. A cade lamb.”
The meaning then extended to refer to any animal kept in a house for pleasure or companionship. Mark Twain uses the word in his Adventures of Huckleberry Finn (1884); “A prisoner’s got to have some kind of a dumb pet, and if a rattlesnake hain’t ever been tried, why, there’s more glory to be gained in your being the first to ever try it.”
In Scotland, the word pet was also to describe a spoiled child or a favorite. Scottish author Sir Arthur Conan Doyle used it in The Memoirs of Sherlock Holmes in 1894; “Dark or fair, she is my own dear little girlie, and her mother’s pet.”
The word took on a more affectionate spin as a term of endearment for someone sweet, obliging, or obedient. By the 20th century, this was a common meaning. For example, P.G. Wodehouse wrote “Do be a pet and go and talk to Jane Hubbard. I’m sure she must be feeling lonely.” (Girl on Boat, 1922) and it has spread from it’s Scottish origins to be in widespread current use across the UK.
By the end of the 16th century, pet had shifted from being just a noun to working hard as an adjective. Here’s where the “pet peeve” and “pet hate” constructions began. At this point in time, it was used to refer specifically to animals, as in a “pet dog” or “pet parrot,” but by the 19th century, it was being used more generically as an adjective to mean something that is “Specially cherished; for which one has a particular fondness or weakness (OED, Vol. XI, p. 626).
I got this from The Etyman(™) Language Blog
