Rules books, that is.
I think if someone did a word cloud for the manuscript for the horse color book, the top words would all be synonyms for “sometimes”. I know what happens if you use the word always or never when talking about horse color; someone will find one that proves the statement wrong.
Last year some very odd grey horses have surfaced that appeared to break the rules – or at least the range of normal – for that color. The first of those was the PRE stallion Comico IV (more pictures are here), followed by his brother Comico VI. Both stallions turned grey unusually late, and also had a pattern of progression that was a bit odd, too. The white seemed concentrated along the dorsal area, which gave the horses a frosty look.
Recently my friend Elaine Lindelef sent pictures of her Connemara mare’s grandsire, *Canal Laurinston. Like the Comicos, he greyed really late. Elaine thought he was probably 17 when she took those pictures. Pictures of him as a mature stallion (age six) can be seen here and here. In that last photo, white hairs are visible on the bridge of his nose, which is an odd area to begin changing color when the rest of his face is still dark. Even in these photos as an older horse, his muzzle appears unusually dark while the bridge of the nose is really pale.
It is also odd that his tail is so dark while the top is light; most greys turn light on the bottom of the tail first.
Comico IV also had that same very pale tailhead.
He is unusual for a grey, but he has relatives that are stranger still. I don’t have permission to post these photos, so links will have to do.
This mare currently being offered for sale is of similar breeding:
The patterns on the two mares look a great deal like reticulated spotting, which is sometimes called lacing, but it is a lot more extensive than I have seen. It also appears more diffused – like the lines of the spots got blurred - than the usual laced pattern. The positioning on the first mare, which is strongly towards the forehand, is unusual as well.
It may be that this particular line has both grey (an extremely slow version of it) and lacing, or perhaps there are other as-yet-unidentified modifiers at work. The end result is certainly odd!