Isidore of Seville [7th century CE] (Etymologies, Book 12, 6:9): Sea-horses [equus marinus] are so called because they are horses [equus] in their front part and then turn into fish. - [Barney, Lewis, et. al. translation]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Marine monsters 6.18, 6.57): [Thomas describes the sea-horse under the names equus maris and zydrach.] [Marine monsters 6.18] The sea horse (equus maris) is a large monster. This monster is the strongest of almost all marine monsters. Nothing can do without water, as Aristotle says; but as soon as nature lacks water, it will also lack life. But the latter part [tail] of the sea-horse ends in the nature of a fish. It lives on the flesh of other fish. In battle it is cruel, but it fears man, and fears that wars will be fought against it. [Marine monsters 6.57] Zydrach is a sea monster of a very strange form, and which pretends malice in its appearance, but is nevertheless harmless. It has a head shaped like a horse's, though of a smaller form; but the body is very similar to the dragon in every respect. It has a tail long in proportion to the size of the body, slender and tortuous like a snake. The whole body has different colors. Instead of wings it has fins like a fish, and it moves from place to place by swimming. - [Badke translation/paraphrase]

Slavic Physiologus [15th - 16th century]: This seahorse [lives] in the sea. He is like a horse in front, very beautiful, and from the waist to the tail he is like a fish. He has a sombre beauty because he goes to the sea and is the sea chieftain of all fish. His glance is like that of a fish. In a certain land, there is a fish, golden in appearance, for everything in it is of gold, and it lies somewhere. The seahorse goes to it, and all fish follow him to that gold fish. The seahorse goes once a year and bows before it as before a king. All fish [do] the same after him and return to their places. The males go forward and [cast] their semen. The females go behind, take up the semen, and are thus inseminated. - [Stoykova, English translation by Mladenova and Stoykov]