The unicorn - a majestic white horse with a single spiralled horn - is one of heraldry's most prevalent beasts. Often depicted with parts of other heraldic animals, such as a goat's beard or lion's tail, the heraldic unicorn can be represented in a variety of attitudes, but is most often found rampant or as an isolated head. Although a mythical creature, the unicorn was believed to be a real animal until the 17th century - unicorn-like creatures are mentioned in the Hebrew Bible and ancient Greek works of natural history. In the middle ages it was believed that the dangerous and courageous unicorn could only be tamed by a virgin, giving the creature a symbolic association with both extreme strength and virtue. Its horn was believed to have healing properties - it was said it could purify poisoned water - and remains a symbol of the medical professions to this day.