These creatures are pets of the four horsemen, unlike their counter parts that can be seen by anyone. If they want them to see them some would appear as normal dogs but if someone tries to threaten their owners then they'll show themselves to that person. While they're hunting down castiel hellhound name Seth has known as the righteous of the sins thanks to cas.

War=firestorm:

Famine=melanin

Disease=kage:

Death=silence:

Castiels hellhound/righteous sins= Seth: