Frosty is where there are intermingled white hairs seen on the bony areas on the horse. These areas include the neck, shoulders, spine and hips. The manes and tails usually have white hairs mixed in. It is said that some Frosty horses are born with manes and tails the colour of the coat however they will gain more and more white hairs in the mane and tail as the horse ages. It is said that the Frosty coat can be seen within the Spanish Mustang breed.
There are a few theories to what causes Frosty horses. It could be another expression of Roan. It could a mutation of Roan (a different allele found at the same locus, such as Agouti). Or it could be something completely different.