[Nancy1999]

Most dog breeds were developed not to be a prized pet, but to do a job and help humans. Yorkies were ratters and worked in coal mines. The miners put them in a tunnel and the Yorkie would kill the rats that lived in the tunnels. The docked tail was use to pull them out of a tunnel, a long thin tail could be too easily broken so they docked them and cut off the skinny tips. If breeders stopped docking tails, some Yorkies would have no problem, but those Yorkies with small bones might be more likely to break their tails. While docking is prohibited in England, working dogs are exempt from the no docking rule.

navillusc, you're correct, they were also used in ratting contests, with the dog killing the most rats, the winner. The AKC has recently developed earthdog games and the rats are now in cages, and all the Yorkies have to do is find them, no killing is involved.