In the equine world, while extremely rare, a chimera is a horse created from two genetically-different DNA types, believed to occur when non-identical twin embryos fuse into one at an early stage of development. While the embryo develops normally, body parts, organs, etc, are made up of different DNA, resulting in a ‘genetic mosaic’ creature whose body contains a mixture of cells of two different genotypes. Chimerism has also been documented in cats and even humans.

Chimeras are often marked by distinctive brindle patterning in the coat. These striking brindle coat patterns are more likely if the twin embryos were bay and chestnut, for example, rather than bay/bay or chestnut/chestnut. While often this unique coat is simply inherited, as is often seen in dogs and cattle, DNA testing of mane and tail hair samples may indicate two distinct individuals, and a blood test can determine if the animal is truly a chimera. Chimeras may also exhibit an unusual bi-coloured or splotchy coat.

Even if the twin embryos were male and female, the chimera will have the reproductive organs of one or the other and should be capable of living a healthy life and producing normal foals. The typical chimera coat patterns cannot be passed on to offspring, as they are caused by an anomaly in the womb, rather than heredity. It is advisable to know what line of DNA the sperm or egg of a chimeric horse carries, however, to best determine the outcome if you plan to use the animal for breeding.