What is the history of the unicorn in literature?

When a mama unicorn and a baby unicorn love each other very much...

The earliest written history mentioning unicorns comes from the ancient Greek author Ctesias. His book Indika ("on India") claimed the existence of these single-horned animals in that land. In his account they were described as being like wild asses with a horn a cubit and half long (a bit over 2 feet, somewhat less than a metre). Note that India was an exotic, unknown, foreign land so about anything could be thought possible and could not be easily disproved.

Various later authors repeated and sometimes embellished this account. Other unicorn-like animals were also claimed to inhabit the Caucasus mountains and areas of Africa. Some artistic representations that appear to be unicorns could just as easily be 2-horned animals represented from the side such that the second horn would be exactly behind and obscured by the first - but someone looking at it might interpret it as a unicorn. Single horns were often passed of as being from a unicorn. Narwhal tusks were particularly popular for this purpose since few people had seen a narwhal and could not imagine an ocean-going animal with a single tusk like that.