You can only encounter them at night. In addition to the fact that they are quite difficult to catch, they have very frightening methods of punishment, coupled with their appearance. Their location can only be found in a specific part of the game world. They will also be associated with one of the most intense missions of the story.
To meet them, Arthur will need to go to Bayou Nwa, located just North of Saint Denis. Then you should wait until nightfall.
After that, you can start completing the task. Arthur must be ready for a firefight, so at this point you need to have at your disposal the best weapons and equipment, otherwise the mission may end with the death of the main character.