c. 1300, "the unicorn," from Old French monoceros "unicorn," from Latin monoceros, from Greek monokerōs, from monos "single" (from PIE root *men- (4) "small, isolated") + keras "horn of an animal," from PIE root *ker- (1) "horn; head." The constellation below the Twins and the Crab is recorded by this name in English by 1797. Probably it owes its origin to Flemish cartographer Petrus Plancius in the 1590s.
This is a modern constellation, generally supposed to have been first charted by Bartschius as Unicornu; but Olbers and Ideler say that it was of much earlier formation, the latter quoting allusions to it, in the work of 1564, as "the other Horse south of the Twins and the Crab"; and Scaliger found it on a Persian sphere. [Richard Hinckley Allen, "Star Names and Their Meanings," London: 1899]