There are many Superstitions about White Horses. Here are a few of the older ones.
1850 N & Q rst ser. I 451 [Midlands] It is believed a sign of 'bad luck' to meet a white horse, unless the person spits at it. 1861 N & Q znd ser. XII 490-I. Grey horses at a wedding lucky. This probably is simply because white is con¬sidered the wedding colour. 1861 C. c. ROBINSON Dialect of Leeds 3 I 6. 'Good luck for a grey horsel'-A common expression of children, accompanied by the act of spitting over their little finger, at the sight of a grey horse, an action supposed to bring good luck, and .. they congregate at places where a long line of road is before the eye, and at the first glimpse of such an animal perform this ceremony. 1882 N & Q 6th ser. VI 9 [Oxon.] Walking the other day .. with some of my schoolchildren, we were met on the road by a carriage and pair of greys. Some of the children at once spat on the ground .. I was told that it was very bad luck to meet a pair of white or grey horses, and that one's bad luck could only be averted by at once spitting on the ground. 1884 N & Q 6th ser.
Edit: There are nearly two pages of superstitions about white horses in two books. Gods and generals or emperors seem to have an obcession for white horses. What can really be interesting is albino horses with "red" eyes and all albino animals. More than 30 that I've found so far were burned to death with so called "witches."
The Dictionary of Superstition