The pre-caudal gland is located at the base of a wolf's tail. It can be used to release a pheromone

Pheromone

A pheromone is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting outside the body of the secreting individual to impact the behavior of the receiving individual...

 onto another wolf, marking that wolf as a member of a particular pack. Usually, this marking is performed by the alpha male.

Descendants of the wolf, many domesticated dogs have vestigial pre-caudal glands, the state of development of which depends on the specific breed. Like wolves, dogs will smell other dogs to discern identifying scents which emanate from the both the anal and pre-caudal glands.