Welsh Corgis are born with naturally short tails. They are one of the few dog breeds that are naturally bobtailed.
The reason for their short tails is a genetic mutation. This mutation doesn't affect the health of the dogs and is considered a breed standard.
It is illegal to dock the tails of dogs in many countries, including the UK and the USA.
So, while the short tail of a Welsh Corgi might look like it was cut, it's actually a natural characteristic of the breed.