It's not likely that Kubanochoerus sightings were the roots of the unicorn legend, because unicorn legends come from Tibet.
In ancient Asia (like India and Tibet), there was once a custom that asked young goats having their horns artificially linked by smiths; they thought those one-horned animals would bring them luck. If you look at ancient artworks, you will see those "tibetan unicorns", hairy goats with either only one horn, or their two horns torsaded one around another. And it seems it's the sighting of those "artificial" unicorns by european travelers that brought the legend of unicorns to life.
You see, if you look at old european unicorn artworks, you will notice that not only THEY ARE WHITE (a color Kubanochoerus musn't have had unless they were leucistic), but that they also have GOAT-LIKE CHARACTERISTICS: a beard, two hooves per foot and a rather small size. It's only later that unicorns started to be represented horse-sized, because of the discovery of narwhal tusks, which were sold as unicorn horns (of course, to have such a big horn, the unicorn needed to be bigger than goats; so, over the years, they were painted bigger and bigger).
So, about unicorns, you have to remember two things:
1- Unicorn legends come from artificially-transformed goats in India (old asian artworks tend to prove it), this is why they were always represented white and goat-sized by Europeans, and
never looked like rhinoceroses or pigs in their descriptions;
2- Narwhal tusks gave the legend more and more life, but that also brought Europeans "reconsider" unicorn size, which concluded in unicorns being painted bigger and bigger over the years.
