Church Grim (this post contains both mythological fact, as well as things I personally have found true in my experiences)
A church grim is a spirit that protects a church. Some people believe these spirits are the spirits of animal sacrifices done in the name of Yahweh (usually buried alive). When a church is destroyed, the church grim becomes a "stray" in the form of either a black dog (ghost) or a hellhound (if it is found by the Devil). These spirits will seek the harm of those who destroyed its church, but will most likely get lost in the processes. These spirits can be found traveling endlessly without a home; and the amount of trouble it gets itself into depends on the unique spirit.
Church grims are more common in Europe, however can be found occasionally in the Americas. The spirit can be found in graveyards where priests have been buried, in abandoned churches, or sometimes near newly built churches. You can also summon a church grim by creating an Abrahamic altar in your home, and giving the spirit a vessel for it to live. Feeding and caring for this spirit is important, and strives off of leftover communion elements or wine and bread. Though most often a black dog, the spirit can be any type of sacrificial animal and occasionally can be white.