Sources : Millago

Isidore of Seville [7th century CE] (Etymologies, Book 12, 6:36): The millago is so named because it flies out over the water. And whenever it is seen flying above the water, the weather is changing. - [Barney, Lewis, et. al. translation]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Fish 7.60): The millagro is a sea-fish which, as Isidore says, flies over the waters whenever the weather is seen to change. It flies over the waves as a sign that happiness has ceased. This, indeed, is contrary to the other fishes and monsters of the sea, which seem to play upon the waters in the rising of the storm. - [Badke translation/paraphrase]