Why is a priest's collar called a dog collar?

Answer 1

The clerical collar, informally called a "dog collar," is used by Anglican vicars and other clergy as a sense of separation between himself and the secular world -- to stand out as clergy.

Answer 2

Dog collar is just a colloqual expression, it is actually known as a clerical collar. It is called a dog collar as it is similar to a dog collar, a priest's collar is exactly the same as a regular detachable collar worn by all men prior to recent times, they became popular in the 19th century as they could be starched much more stiffly then the rest of the shirt. The only difference with clerical collars is that they are worn backwards - with the overlapping, open part being concealed by the back collar of the jacket - thus they have a continuous collar around their neck from the front view - like a dog's collar.