How do Unicorns usually die?

The unicorn is a solitary animal, rarely associating even with others of its own kind. Its scarcity, reclusive nature and swift speed make it an extremely difficult creature to observe, let alone kill or capture. In fact, this animal is so skilled at eluding hunters that it has given rise to the legend that only those pure in heart are capable of capturing it. The method for hunting unicorns recommended during the Middle Ages was for the hunters to bring an innocent young maiden into the forest to lure the unicorn out of hiding. When the unicorn was lulled to sleep, the hunters would spring from hiding and remove its horn. Invariably, without its horn, a unicorn will die.

http://members.tripod.com/rc-anodizing/PEZT/animalsZT/unicorn.htm

Here is a concrete way of telling them apart.....

https://en.wikipedia.org/wiki/Invisible_Pink_Unicorn