The Unicorn is the name of several supervillains appearing in American comic books published by Marvel Comics.

The first Unicorn (Milos Masaryk) debuted in Tales of Suspense #56 (Aug. 1964) and was created by Stan Lee and Don Heck.

Milos Masaryk is a Soviet intelligence agent assigned to track down the original Crimson Dynamo, who had defected to the US. Wearing technology designed by the Dynamo and calling himself Unicorn, Masaryk battles Iron Man, who defeats him. The Unicorn is among the villains affected by Doctor Doom's high-frequency emotion charger, compelling him to attack the Fantastic Four at the wedding of Reed Richards and Susan Storm.

The Unicorn later allies with Count Nefaria and undergoes experimental conditioning to augment his powers. The process leaves Unicorn with "accelerated cellular deterioration", and he is taunted with a cure by the Red Ghost in exchange for assistance against Iron Man. Once it becomes evident that the Red Ghost has lied, the Unicorn aids Iron Man and later escapes.

The Mandarin makes a similar promise to the Unicorn and deploys him against Iron Man on two separate occasions, both ending in Unicorn's defeat. In the first instance, the Mandarin's consciousness becomes trapped in the Unicorn's body before being freed. Using the alias "The Other", the Titanium Man uses the Unicorn against Iron Man, who is finally able to capture him. In a dramatic turn, the very moment of his capture sees Masaryk slip into a life-threatening coma. Iron Man takes Masaryk to the Avengers Mansion, where teammate Yellowjacket wakes Masaryk and cures him of his debilitating disease. The process has the result of driving Masaryk insane, and a disorientated Unicorn inadvertently activates the hidden robot Arsenal. Unicorn is stunned by Arsenal — which Iron Man drives off — and then placed in stasis pending a cure for his mental illness.

Months later, Masaryk is freed when a fire breaks out at Stark Enterprises. He resumes the identity of the Unicorn and finds and attacks Iron Man once again. Still insane, the Unicorn refuses to believe that "The Other" was a lie, and begins to walk back to the Soviet Union via the ocean to find him. A severely weakened Iron Man watches as the Unicorn apparently drowns.

The Unicorn is later revived by the entity the Beyonder to fight with a new version of the Lethal Legion. The Unicorn is given a true third eye on a stalk with energy projection capabilities in lieu of a suit.

During the "Infinity" storyline, Unicorn is among the villains recruited by Spymaster to help him attack the almost-defenseless Stark Tower.