For the man of the Graeco-Roman world the dog was essentially a chthonic beast, he was, one might say, the earthly embodiment of the demonic. Hecate was ruler of dogs and demons and appeared in canine form
The verses of Theocritus II 10ff, in which the sorceress whispers in the lonely moonlit night are well known:
Up then, Selene and give me good light, thou silent goddess.
To thee now I sing, and to thee also, Hecate thou dost live
Thou dost frighten away whimpering dogs when thou dost walk
Over the graves of the dead and over the dark blood-stream
Hail to thee, Hecate goddess of horror.
Here is a prose translation of some verses of Synesius which shows us very vividly how people in the Graeco-Roman times, even when they had become Christians felt that this dark submerged part of the soul was threatened by the hounds of Hecate and saw in this a symbol of the devil:
“Let the sinuous trend of serpents sink beneath the earth, and that winged serpent also, the demon of matter, he who clouds the soul, rejoicing in images and urging on his brood of whelps against my supplication. Do thou, O Father, O Blessed One, keep away from my soul these soul-devouring hounds, from my prayer, from my life from my works.”