"furnished with horn or horns," Old English hyrned, from source of horn (n.). The modern word probably is a new formation in Middle English. From late 14c. in reference to Moses, and the horn-like rays of light, symbols of power, that appeared on his head as he descended Mount Sinai. From 1620s in reference to cuckolds. Horned toad is from 1766; horned question is "a dilemma" (1540s).
The HORNED TOAD is frightful ; his head half the size of his body ; his jaws open enormously ; his eye lids have the form of a pointed cone, which makes them seem armed with horns, wherein are his eyes. His feet have something the air of hands. [Francis Fitzgerald, "The General Genteel Preceptor," 1747]