Sorry for the terrible name; didn't know what to call these.
Backstory: an eccentric multi-billionaire decided that friesians were lacking in colour. He wanted to keep the build and temperament of a friesian, but with the white markings of other horses. And he wanted results within ten years. The horse-breeding community would have nothing to do with this, but a man named Dr Arden, a disgraced genetic engineer, decided to take on the challenge. But Arden was disgraced for a reason; his mad ideas and strange splices had been dubbed 'crimes against nature'. He set about isolating the correct genetic code for chrome (white markings) and placing it on friesian horses. The first batch of foals showed nothing. The friesian gene was too dominant. The billionaire threatened to cut off Arden's funding.
So Arden got creative.
The foals that emerged were glorious; pitch black, curly-maned, and with the most exciting white markings the billionaire had ever seen. They had spots and stripes and swirls and dashes; dapples like baby deer and rosettes like panthers. They were quite extraordinary. And, for a while, all was well.
Until the billionaire came to visit his foals one evening with a friend. And realised that their white markings glowed in the dark. He was furious, of course, but Arden soon smoothed things over, calling it a temporary measure, which would fade with age. Only, it didn't. It got brighter. And the foals grew strange but beautiful netting, under their forelegs and over their curly tails. At this point, the billionaire insisted on seeing Arden's methods. He had spliced glowing fungus genes onto horses, in the hopes of producing 'flashier' markings. The billionaire was appalled, and called for an end to the project. Arden agreed, and left.
Only, there have been strange friesian foals cropping up on the market lately, all with flashy white markings.