So. I have found myself thinking about unicorns. As with most thinking, it is useful to start with what we know. There are mediaeval reports of unicorn sightings, but there are no sightings today. We also know that unicorns could only be tamed by a female virgin. That is all we know.
The fact they could only be tamed by a female virgin suggests unicorns are male. Are all unicorns therefore male, or do both male and female of the species have horns? If we consider peacocks and creatures such as deer, we find it is the male that has the sexual display characteristics for attracting mates and in some cases the horns for fighting competing males. If unicorns follow the same pattern, then the horned animals are the males, and the unnoted, hornless animals are the females.
What do these hornless creatures look like? They look like horses. In fact, they are horses.
The mediaeval naturalists were ignorant of genetics, so did not understand how recessive genes can produce features in a child which are not manifested in the parents. The simplest explanation of the disappearance of the unicorns is that they were actually male horses affected by the horn-bearing gene. Any half-way competent geneticist should be able to demonstrate how a deeply recessive horn-forming gene could have been lost from the horse genome since the early middle ages.
So there we are. Unicorns solved. Next.