Not just a purple saddle, or Prince-pleasing purple reins. But an entirely purple mount.
I'm not certain that nature works this way, but I guess when you're a Witcher, you can bend nature to your will. According to Bitcoin Boffin, the bug happened when downloading the Temerian Armour set for Roach prior to starting the game, but it's not clear if the file fully downloaded prior to playing, or something else that could be patched out has occurred.
Thanks to Emanence for the tip!