Boyd’s Bible Dictionary:
(one-horned). A fabulous animal pictured as having one horn on its forehead and the body of a horse. The Hebrew word re’em, which is translated “unicorn” (Num. 23:22; 24:8; Deut. 33:17; Job 39:9; Psa. 22:21; 29:6; Isa. 34:7), does not refer to the one-horned creature of fable, but evidently to a two-horned animal (Deut. 33:17), possibly the now nearly extinct wild ox, auroch or urus of naturalists.
Concise Bible Dictionary:
The Hebrew word is reem, translated in the LXX by ἁδρός, and μονοκέρως, from the last of which the AV adopted the rendering “unicorn.” There is nothing in the scripture to intimate that the animal had but one horn, indeed Deuteronomy 33:17 speaks of the horns of a “unicorn” (see margin), and it must have been some animal with which the Israelites were familiar. Its great strength and untamableness are the main characteristics: it cannot be utilized, as the tame ox, for agricultural purposes (Num. 23:22; Num. 24:8; Deut. 33:17; Job 39:9-10; Psa. 29:6; Psa. 92:10; Isa. 34:7). The Lord asked to be saved from the lion's mouth, for Jehovah had heard Him from the horns of the unicorns (Psa. 22:21), to which His implacable enemies are compared. It is most probable that a species of wild ox, the Bos primigenius, is referred to; these may have been known in Palestine, as was the lion, though they are now extinct. This is confirmed by the wild ox being sculptured in an Assyrian bas-relief, with the name reem or rim over it.
Strong’s Dictionary of Hebrew Words:
Meaning:
or rieym {reh-ame'}; or reym {rame}; or rem {rame}; from 7213; a wild bull (from its conspicuousness)