Beliefs like this led to a strong European market for unicorn horns, and in the Middle Ages opportunistic sailors started selling narwhal tusks as unicorn horns. Before that, says the American Museum of Natural History, European unicorns often had stubby and/or colored horns. After that, the horns were "long, white and spiraled."
Western unicorn mythology brings us somewhat closer to the modern myth. European unicorns often have white coats, a horse's body, the hooves and beard of a goat and the tail of a lion. These unicorns are nearly impossible to catch, a trait credited to strength or general elusiveness. But they do have a weakness.
A virgin woman can lure the European unicorn into the open. She seems to entrance the creature, who may lay its head in her lap — by some accounts suckle at her breast — leaving itself vulnerable to capture by hunters waiting out of sight.
This association with the virgin, along with reported biblical mentions and the abilities to heal and counteract poison, led the medieval Christian church to cast the unicorn as a Christ figure. The creature thus increasingly came to represent purity and nobility, likely contributing to modern representations of the unicorn as benevolent, regal, graceful and white.
How it became the sparkly, smiling creature of popular culture, seen in the works of Lisa Frank, the My Little Pony empire and farting-rainbows memes is not entirely clear, but it probably has to do with commercial value. Kids are drawn to unicorns. Their parents buy them unicorns. Single-horned chimeras that impale bad people with their horns likely wouldn't fly in the 6-year-old set.
Where the Indus Valley unicorn fits into known unicorn legend remains a mystery. That its image appears on more than a thousand seals recovered by archaeologists suggests it was highly valued. It may have been sacred. It may even have been real. The Indus unicorn will keep its secrets until science finds the key to the ancient code.