Ovid [1st century CE] (The Metamorphoses, Book 13, 705-737): Scylla attacks from the right-hand coast, restless Charybdis from the left. The latter sucks down and spits out ships she has caught: the former has a girdle of savage dogs round her dark belly. She has a girl’s face, and if the tales of poets are not all false, she was once a girl also. Many suitors wooed her, whom she rejected, and she would go and tell the ocean nymphs, being well loved by the ocean nymphs, of the thwarted desires of young men. - [Kline translation]

Isidore of Seville [7th century CE] (Etymologies, Book 11, 3:32; 13, 18.3-4): People tell of Scylla as a woman girded with the heads of dogs, with a great barking, because of the straits of the sea of Sicily, in which sailors, terrified by the whirlpools of waves rushing against each other, suppose that the waves are barking, waves that the chasm with its seething and sucking brings into collision. [Book 13, 18.3] It is exceedingly narrow, dividing Sicily from Italy by a distance of three Roman miles, and is notorious for fabulous monsters; on one side appears Scylla, and on the other Charybdis. [Book 13, 18.4] Indeed, neighboring people give the name Scylla to a rock jutting over the sea that is similar to the fabled shape when seen from a distance. Accordingly they have attributed to it the monstrous appearance of the story, as if it had the shape of a human girded with the heads of dogs, because the current rushing together there seems to produce the sound of barking. - [Barney, Lewis, et. al. translation]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Marine monsters 6.46): Scilla is a sea monster like the siren. Andelmus the philosopher truly says that this monster dwells in that sea which is between Italy and Sicily. Indeed, she is an enemy to the sailor and to all men, and delights in their blood and flesh. Her head and breast are, of course, shaped like a maiden, like sirens; and she has a grin and gaping mouth and horrible teeth and a belly like a beast and a tail like a dolphin. They are of wonderful strength and are not easily conquered in the water; but on earth they are less strong, and almost peaceful. Andelmus: They also have somewhat musical voices, and they are also wonderfully pleased with music. - [Badke translation/paraphrase]