It is bordered by Orion to the west, Gemini to the north, Canis Major to the south, and Hydra to the east.

V838 Monocerotis, a variable red supergiant star, had an outburst starting on January 6, 2002; in February of that year, its brightness increased by a factor of 10,000 in one day.

After the outburst was over, the Hubble Space Telescope was able to observe a light echo, which illuminated the dust surrounding the star.

[12] German astronomers Heinrich Wilhelm Olbers and Ludwig Ideler[13] indicate (according to Richard Hinckley Allen's allegations) that the constellation may be older, quoting an astrological work[14] from 1564 that mentioned "the second horse between the Twins and the Crab has many stars, but not very bright"; these references may ultimately be due to the 13th century Scotsman Michael Scot, but refer to a horse and not a unicorn, and its position does not quite match.

Astronomer Camille Flammarion (died 1925) believed that a former constellation, Neper (the "Auger"), occupied the part of that sky now deemed Monoceros and Microscopium, but this is disputed.

[15] Chinese asterisms Sze Fūh, the Four Great Canals; Kwan Kew; and Wae Choo, the Outer Kitchen, all lay within the boundaries of Monoceros.