I had a chimerism test performed to see if his brindling was a result of embryo fusion...
The problem with that is the results are not always accurate. Chimerism is tricky.
When two genetically separate beings absorb together to become one creature, their DNA does not combine into one. The animal will have two different sets of DNA. You are not really able to do a test on blood or hair or any kind of body sample individually and get both
sets of DNA. The two sets spread out to form the different parts of the body. Your horse's tail hair could all be from DNA1 and his lungs and kidneys could be the only parts of his body that DNA2 formed for all anybody knows. The only way to rule out
chimerism is to get absolutely everything
tested (blood, tissue samples from all organs, skin samples, hair samples from mane, tail, and
various areas of the body).
To put this in perspective, I read an article somewhere that a family had a paternity test done on their child. The husband was the actual father of the child, but the DNA didn't match so the test showed that the husband wasn't
the father. It turned out that the man had chimerism and his absorbed brother's DNA had developed to create the reproductive parts of his body.