Sources : Sea-dog

Pliny the Elder [1st century CE] (Natural History, Book 9, 70): The number of dog-fish [canicularum] specially swarming round sponges beset the men that dive for them with grave danger. These persons also report that a sort of cloud thickens above their heads - this a live creature resembling flatfish - pressing them down and preventing them from getting back, and that because of this they have very sharp spikes attached to cords, because the clouds will not withdraw unless stabbed through in this way. This story being the result, as I believe, of darkness and fear; for nobody has ever heard of any such creature in the list of animals as the 'cloud' or 'fog,' which is the name the divers give to this plague. Divers have fierce fights with the dog-fish [caniculis]; these attack their loins and heels and all the white parts of the body. The one safety lies in going for them and frightening them by taking the offensive; for a dog-fish is as much afraid of a man as a man is of it, and so they are on equal terms in deep water. When they come to the surface, then the man is in critical danger, as the policy of taking the offensive is not available while he is trying to get out of the water, and his only safety is in his comrades. These haul on the rope tied to his shoulders; this, as he carries on the duel, he shakes with his left hand to give a signal of danger, while his right hand grasps his dagger and is occupied in fighting. Most of the time they haul gently, but when he gets near the boat, unless with a quick heave they suddenly snatch him out of the water, they have to look on while he is made away with. And often when divers have already begun to be hauled up they are snatched out of their comrades' hands, unless they have themselves supplemented the aid of those hauling by curling up into a ball. Others of the crew of course thrust out harpoons, but the vast beast is crafty enough to go under the vessel and so carry on the battle in safety. Consequently divers devote their whole attention to keeping a watch against this disaster; the most reliable token of safety is to have seen some flatfish, which are never found where these noxious creatures are on account of which divers call them the holy fish. - [Rackham translation]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Marine monsters 6.13): The dogs of the sea [canis maris] are marine animals, as Pliny says, terrible in fury and hostility. For they are enemies to every living thing that yield to their blows. They have the strongest spikes shaped just like nails. They hunt the multitudes and flocks of fish through the sea in like wild dogs that hunt on land, except that they cannot bark, but instead of barking they have a terrible breath. Therefore, following the flocks of fish, they crowd them into narrow places, and thus they are crushed to a cruel death. The fishermen, however, noting the places in which the fish hid from flight, caught them surrounded with nets. This beast can be killed with difficulty by many tridents. - [Badke translation/paraphrase]