show one’s horns To reveal one’s evil intentions; to expose one’s malicious, venomous, or insidious nature. This expression alludes to the horns commonly portrayed on the forehead of Satan, an attribute which also gave rise to one of the devil’s nicknames, Old Hornie.

show the cloven hoof To reveal a treacherous nature or evil intentions. The allusion is to the cloven hoof of Satan, long representative of evil. Although the simple term hoof was in use in this figurative sense as early as 1638, the phrase did not appear until much later.

son of Belial A thoroughly evil and despicable person; the embodiment of wickedness; the devil. This expression originated in the Old Testament (I Samuel 2:12):

Belial, apparently derived from the Hebrew b’li ya’al ‘without use,’ became the equivalent of Satan in later Jewish writings. Belial was also employed as a name for one of the fallen angels in John Milton’s Paradise Lost (1667). The expression maintains its theological application for the personification of evil.