Sources : Vergiliales

Pliny the Elder [1st century CE] (Natural History, Book 9, 33): There are two lakes in Italy at the foot of the Alps, named Como [Larius] and Maggiore [Verbannus], in which every year at the rising of the Pleiades [vergiliarum, the star Virgilia] fish are found that are remarkable for close-set and very sharp scales, shaped like shoe-nails, but they are not commonly seen for a longer period than about a month from then. - [Rackham translation]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Fish 7.92): Vergiliales may derive their name from the time of their appearance, as Pliny says. There are two lakes in Italy at the foot of the Alps, which are called Lakes Larius and Verbarinus. In these lakes fish are predicted to appear to men every year only at the rising of Vergiliarum [Pleiades, the star Virgilia]. But at other times they are hidden. They are remarkable for their dense and sharp scales which are like pointed nails. Their appearance resembles shoes, except that they have the head of a fish. In the head, that is, in the beginning, the shoes are usually small and narrow, but in what follows they are greatly enlarged [Here Thomas probably misread Pliny's description of their scales as being shaped like shoe-nails to mean the fish were shaped like shoes]. By these are signified certain wicked men, who in time of peace are not seen, nor do they appear to men as they are. But since in the time of the Pleiades, in which sometimes floods suddenly occur, and the time of tribulation has come, then their cruelty seems as though it had teeth; and if at first, that is, at the beginning of their existence, they were seen as small, that is, humble, but later on in the time of tribulation they are spread out in wickedness. Whence Solomon: There is one who humbles himself unworthily, and his inward parts are full of deceit. - [Badke translation/paraphrase]