Unicorns are elemantals (along with the fae, merfolk etc.)
The horn of a unicorn is said to have magickal qualities eg: to purifie bodies of water. When humans discovered this quality, it was kept secret by the druids and priestesses in order to protect the unicorn from those who would seek to use its power currishly. But the secret was eventually discovered, hunters were sent to find and kill the unicorns so their horns could be used and those who obtained it to used it to then ensure their own food and drink was pure and that any poisons were neutralized.
The unicorns horn is called the alicorn and on many prescriptions and lists of apothecary ingredients the alicon appeared until the mid 17th century when the unicorn was an endangered magickal creature.
Unicorns are shy, rare and wild in nature, and because of this they fled deeper and deeper into the forest, until such time when maidens were used as bait for unicorns (because they are pure in intent, loving, innocent, wild and free, they attract unicorns) and they were able to be hunted again.
There is more evidence of unicorns in Lucy Cavenish's book White Magic.