Interesting question!
As has been said, there have been *many* examples of "fake" or unofficial names applied to diesel locomotives. As far as shunters go, I wouldn't like to guess as to the number applied since the early seventies, mostly by stencil or paintpot of the home depot. Some lasted years, others days or even hours (anyone remember "Percy The Pilot"?!)
Other shunter names were applied at a yard outbase by a member of traincrew or local wag and were either quickly painted over at the depot during the next visit for exam, or left in situ, becoming semi- official. These were often local names significant to the home depot: "Bordesley Yard", "Penwithers" etc. I remember 08296 and 177 becoming "Rat 1" and "Rat 2" at Goole (while outbased from BG) and being painted out rather sharpish (as I suspect was "A**e Bandit" at AN!) There were many, many others.
As for main line traction, this could well have started when some class 40s were daubed with simple, white- painted names by lads from the Wirral at Birkenhead depot in the early '80s. These were completely new names such as "Panther" and "Warrior". Other 40s had their original names re- painted (really nicely!) by a local decorator and enthusiast while stabled at Penmaenmawr quarry in North Wales:
https://www.flickr.com/groups/cfas/discuss/72157623456067202/
Next were some class 25s who had stencilled white names applied (while on Cambrian line duties IIRC): "Tamworth Castle" was one of these. This actually became an official name when the loco was reinstated for charter use by InterCity in 1989 and was the longest lived of these "castle" names.
As has been said, Tinsley started its own unofficial naming game with their class 45s in 1987, of which there were dozens. After their withdrawal, they continued with at least one class 31 ("Phoenix") and many more class 47s. IMHO, they did way too many of these and wore out the novelty. Ah well.
Other depots also had their own "pets" and very unofficially adorned locomotives, notable examples being from Thornaby, Stratford and Crewe Diesel. It seemed that when BR blue's stranglehold was finally broken in the late 1980s, the floodgates opened...
