Those who practice magic.
When conjurers and sorcerers worked their magic, they established an area around themselves with a radius of ten or so feet in order to have the best chances of success. This was known as their warded zone. Since their concentration might be disturbed, and, in extreme cases, their spell might even lose its efficacy if someone were to step into this zone while they were working, sorcerers conjured up creatures and set them as watch dogs outside the warded zone, ready to attack intruders. The task often fell to massive hounds, poisonous toads, and serpents suckled on pure malice.