Unicorns are leonine when combined- they were designed to be apex predators when originally created. Unfortunately, this lion aspect may be part of what caused their disappearance. It tied the Contract to a recessive gene, meaning progressive generations were born without the capability to combine and without even a horse aspect at all. This may have contributed in part to the vanishing of the unicorns, while pegasi and kelpies thrived as their Contracts were tied to dominant genes (though in the case of pegasi, only passed on maternally) meaning successive generations were able to mix with non-slipskin populations and still create new slipskins. The unicorns were thought to be extinct by the end of the Wars. However, shortly after their extinction, something appeared and began hunting and killing massive amounts of equine slipskins for years, driving numbers down signifcantly. The death toll was bad enough it drove the most desperate herds to the Cities for refuge. Some slipskins allegedly made it in, but were forced to leave their equine aspects behind, leading to rare sightings of lone half-pegasi and kelpies roaming outside the City walls, driven to madness by the seperation from their human aspect.

The Dark Horse appeared about five years after the predation of equine slipskins suddenly stopped. Perhaps he was in hiding from the mysterious hunter, lurking in the relative safety of the Undergrowth until the predator vanished? Who knows.