In these church walls a live animal could once have been walled up. It could have been a rooster, a cat, a dog or any other domesticated animal living close to people. The animal thus used would from then on be the guardian of the church, its denizens and its environs. If no animal should be entombed in this fashion, the guardian role would instead fall upon the shoulders of the first person to be buried in the churchyard.
The guardian spirit, called the Church Grim, would chase away visitors with evil or mischief in their hearts, be they man, fey or creature of the dark, and would also keep the peace among the regular church-goers. It would take the shape of a big, dark and twisted version of the animal which once gave rise to it and could put fear into the hearts of even the most hardened criminal or monstrous creature of undeath.
Similar practices can be found in many other cultures and are based on the assumption that evil should be expelled with evil (fight fire with fire, so to speak).