Chimera

chimera [ki-MEER-uh]
n. a grotesque product of the imagination

[Derives from Greek chímaira ("she-goat")]

In Greek mythology, the Chimera was a monstrous creature of Lycia (on the southern Turkish coast, east of Rhodes), composed of the parts of multiple animals: the body of a lioness; a tail that ends in a snake's head; and the head of a goat on its back.

In contemporary usage, a chimera is anything that people irrationally fear.

In Secrets: A Memoir of Vietnam and the Pentagon Papers, Daniel Ellsberg classifies John Kennedy's claim that in 1960 the United States was dangerously behind the Russians in the nuclear arms race as an irrational fear:

The missile gap favoring the Soviets had been a fantasy. There was a gap all right, but it was ten to one in our favor. … The specter of a deliberate Soviet surprise attack suddenly appeared, with the new estimates, to have been a chimera.