Sources : Monocerus

Pliny the Elder [1st century CE] (Natural History, Book 8, 31): Pliny does not distinguish between the monocerus and the unicorn.

Gaius Julius Solinus [3rd century CE] (De mirabilibus mundi / Polyhistor, Chapter 52.39): But the cruellest of all is the monoceros, a monster which roars horridly, has the body of a horse, the feet of an elephant, the tail of a pig, and the head of a deer. [40] A horn of wonderful lustre sticks out of its forehead, to a length of four feet. It is so sharp that whatever it attacks is easily pierced by its blows. It does not come alive into the power of men. It may, indeed, be killed, but not captured. - [Arwen Apps translation, 2011]

Aberdeen Bestiary [circa 1200 CE] (folio 15r):The monoceros is a monster with a horrible bellow, the body of a horse, the feet of an elephant and a tail very like that of a deer. A magnificent, marvellous horn projects from the middle of its forehead, four feet in length, so sharp that whatever it strikes is easily pierced with the blow. No living monoceros has ever come into man's hands, and while it can be killed, it cannot be captured.