The animosity which existed between the lion and unicorn is referred to by Spenser, and is allegorical of the animosity which once existed between England and Scotland:
“Like as a lyon whose imperiall powre
A proud rebellious unicorne defyes.”
Faerie Queen, ii. 5.
Mediæval Conception of the Unicorn
The mediæval conception of the unicorn as the water-conner of the beasts was doubtless suggested by that belief of earlier ages which made the unicorn not merely symbolical of virtue and purity, but the more immediate emblem of Christ as the horn of our salvation (Psalms xcii. 10 and lxxxix. 17, 24), expressly receiving its general fulfilment in him (St. Luke i. 69). The horn, as an antidote to all poison, was also believed to be emblematical of the conquering or destruction of sin by the Messiah, and as such it appears in the catacombs at Rome. The unicorn is the companion of St. Justiana, as an emblem betokening in the beautiful legend her pure mind, resisting all the Geraldine-like dreams sent by magic art to haunt her, till she converted her tormentor himself.
He is remarkable, say the old writers, for his great strength, but more for his great and haughty mind, as he would rather die than be brought into subjection (Job xxxix. 10-12).
It was believed the only way to capture him was to leave a beautiful young virgin in the place where he resorted. When the animal perceived her, he would come and lie quietly down beside her, resting his head upon her lap, and fall asleep, when he would be surprised by the hunters who lay in wait to destroy him.
The Legend of the Unicorn.