Unicorns could only be captured by a trap. The trap bait had to be a female virgin or maiden, preferably beautiful. It seems the magical creatures could sense that somehow.
When the unicorn came upon the “fair maid” sitting alone–apparently–he couldn’t help himself. He had to approach her and put his head in her lap where he promptly fell asleep. The maiden would usually bind him with her own girdle.
The hunters would advance from behind the trees and bind the poor unicorn. They would put it in a zoo or menagerie after sawing off its horn. The horn would be made into an anti-poison wine cup for royalty.
Even the wild unicorn couldn’t keep its wits around a beautiful woman!