Named almost literally, chimera is the fusion of two separate organisms (in this case fraternal twins). Chimeras occur when two embryos fuse early in pregnancy, the color genetics of each one plays a factor in the animal’s coat. This can (and often does) manifest itself as brindle, but on rare occasions the animal will have patches that look like they come from a different color horse.
The Pretty Kind of Mutant
This is a color characteristic that is not genetic. The change occurs after conception, so DNA found in reproductive cells is not responsible for the unique coloring. Chimeras cannot pass this patterning to their offspring, as they are essentially mutants (not in a bad way, of course). As this is the case, chimerism can likely be found on any breed and in any location. Unless there are specific environmental elements that cause this mutation in pregnant mares.
We consider ourselves fortunate to have found so many images of what we hope are chimeras. Under each we’ve taken a stab at the two different base colors involved.
Chestnut (cream) + Black
Grey + Brown
Grey + Black
Chestnut (cream) + Bay