I found two places in Tertullian (there may be others) where he writes about "horns of the unicorn" being related to the cross:
1. An answer to the Jews:
http://www.earlychristianwritings.com/t ... ian08.html
2. Against Marcion, Book 3:
http://www.earlychristianwritings.com/t ... an123.html
Both follow the same idea in Justin Martyr's writing. I've quoted from Tertullian's "Against Marcion", Book 3, below. It's interesting, because Tertullian gives several examples of how the crucifixion could be inferred from various passages in the OT:
First, then, Isaac, when he was given up by his father as an offering, himself carried the wood for his own death. By this act he even then was setting forth the death of Christ, who was destined by His Father as a sacrifice, and carried the cross whereon He suffered. Joseph likewise was a type of Christ, not indeed on this ground (that I may not delay my course), that he suffered persecution for the cause of God from his brethren, as Christ did from His brethren after the flesh, the Jews; but when he is blessed by his father in these words: "His glory is that of a bullock; his horns are the horns of a unicorn; with them shall he push the nations to the very ends of the earth,"--he was not, of course, designated as a mere unicorn with its one horn, or a minotaur with two; but Christ was indicated in him--a bullock in respect of both His characteristics: to some as severe as a Judge, to others gentle as a Saviour, whose horns were the extremities of His cross. For of the antenna, which is a part of a cross, the ends are called horns; while the midway stake of the whole frame is the unicorn. By this virtue, then, of His cross, and in this manner "horned," He is both now pushing all nations through faith, bearing them away from earth to heaven; and will then push them through judgment, casting them down from heaven to earth. He will also, according to another passage in the same scripture, be a bullock, when He is spiritually interpreted to be Jacob against Simeon and Levi, which means against the scribes and the Pharisees; for it was from them that these last derived their origin. Like Simeon and Levi, they consummated their wickedness by their heresy, with which they persecuted Christ. "Into their counsel let not my soul enter; to their assembly let not my heart be united: for in their anger they slew men," that is, the prophets; "and in their self-will they hacked the sinews of a bullock," that is, of Christ. For against Him did they wreak their fury after they had slain His prophets, even by affixing Him with nails to the cross. Otherwise, it is an idle thing when, after slaying men, he inveighs against them for the torture of a bullock! Again, in the case of Moses, wherefore did he at that moment particularly, when Joshua was fighting Amalek, pray in a sitting posture with outstretched hands, when in such a conflict it would surely have been more seemly to have bent the knee, and smitten the breast, and to have fallen on the face to the ground, and in such prostration to have offered prayer? Wherefore, but because in a battle fought in the name of that Lord who was one day to fight against the devil, the shape was necessary of that very cross through which Jesus was to win the victory? Why, once more, did the same Moses, after prohibiting the likeness of everything, set up the golden serpent on the pole; and as it hung there, propose it as an object to be looked at for a cure? Did he not here also intend to show the power of our Lord's cross, whereby that old serpent the devil was vanquished,--whereby also to every man who was bitten by spiritual serpents, but who yet turned with an eye of faith to it, was proclaimed a cure from the bite of sin, and health for evermore?