His name was Rango and he was born a sorrel pinto. Both sire and dam were sorrel pinto's, but he started to darken as he aged. He turned into a black pinto and even sired some black pinto foals...How did this happen?
This is not supposed to happen so I hear, what is your opinion?
First off, foal colors can be almost meaningless when it comes to the eventual adult color-- many foals are born looking one way and then shed off in an unexpected direction.
Were either of his "sorrel" pony parents red with a flaxen mane and tail? If so, they might have been a bay with the silver gene (commonly called silver dapple, taffy, etc). Silver dilutes black pigment and leaves the red pigment alone, so when a bay has the silver gene, the body stays red, but the black mane and tail and legs are much lighter.
Although there are usually some visual "clues", a silver bay can be pretty easily mistaken for a flaxen chestnut/sorrel. Silver is pretty common is Shetlands, and some other pony breeds, and also in Miniatures.
If either of his parents were actually a silver bay, it would explain how they were able to produce a black foal-- the bay contributed the E for black pigment, and the foal didn't inherit an "A" for restricting black to the points, nor did it inherit silver for diluting any black which was present.
Here are a few silver bays (pinto and solid), for reference--
Another possibility is that he is actually a very very dark liver chestnut. He looks black to me, but there are some chestnuts so dark that they look very black.
Depending on what his parents looked like, I think the silver/bay parent explanation is more likely, as he does look like a "real" black....
Yes I have seen many Miniature Horses and Shetland Ponies who were so variation of a silver dapple bay. My problem is both sire and dam are deceased and there are very few pictures of them. No sure about the sire....
But I do think the dam might be a silver dapple bay..what do you think?
I think the dam might be silver as well. On a side note I've seen silver mess with mini's often...Take my old gelding, he's genetically a black silver dapple. When we bought him as a weanling he was DANG near white with dark shoulders and butt when we sold him he was blue..almost black.
The first picture you can BARELY seen him, but he's the white blob in the back behind the brown, and the second picture is before we sold him at 5 years old. The last picture is from when he was 3 years old, so you can see the color changes there.