Sources : Narwhal

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Marine monsters 6.34): Monocheros is called a sea monster, which has its name from this: for 'monos' is interpreted as 'one', 'ceros' as 'horn'. Whence the monocheros is called a one-horned beast (as the scriptures refer to the wild land animal as a unicorn). It has a most terrible horn in front, with which it can certainly penetrate and devastate ships it encounters and kill multitudes of men. But in this the mercy of the Creator took counsel from the human race, because, since the cruelest animal is created the slowest, ships can escape at the sight of the monster. - [Badke translation/paraphrase]