From 1908 to 1925, Firehorse Fred pulled the firewagon in New Bern, North Carolina. On the average, horses live 20 to 30 years, so Fred must have been quite worn out when he died in harness aged about 17. At the time, he was pulling the firewagon to a false alarm.
Well loved by his firehouse crew and by the townspeople, Fred came in for particular attention: his head was stuffed and mounted trophy style and installed in New Bern's Fireman's Museum. And there he stays.
Firehorses seem to have done duty as late as 1929, if the articles in firehorses.info are any indication. Wonderful historical pictures there too. Many of them retired with honor from their working lives to nice pastures and ease.
They're a borderline type of pet, I think: their primary role was functional, but their situation was one of mutual trust and affection. What defines a pet to you?