Forced DE-evolutionary virus? Turning evolution on it’s heels one mutant at a time

The Forced Evolutionary Virus, also known as FEV, is a virus artificially created by West Tek's Nuclear, Biological, and Chemical Division.  As the name suggests, the FEV was developed more or less by accident - it started as an immune system booster to make American troops resistant to the biological weapons they were sure China was developing because America sure as hell was. As time went on however, they found it also turning test subjects into musclebound giants, so they changed the name and started pursuing its potential as a super soldier serum instead, using it to forcibly advance a genome into its next distinct stage of evolution. But when a human is exposed to it they gain incredible amounts of physical strength and endurance but they regress in intelligence and many become completely infertile (such as in the case of a Super Mutant).
This is because a notable feature of FEV is that it partially relies upon the DNA of the individual coming in contact with it in order for it to determine the course of action it should take. Coupled with portions of its own recursive code, effects of exposure can be unpredictable, with many resulting in the creations of various creatures, with prolonged immersion in the virus resulting in the most stunning transformations, such as the super mutants that live in both the west and the east, as well as the Kronenberg monstrosities dis-affectionately dubbed centaurs to all the new creatures such as the Snallygaster that call Appalachia home and dot it’s ever expanding wilderness. 
However, an interesting, yet seemingly unheard of effect of the FEV is the introduction of an evolution working backwards, resulting in the de-evolutionizing of it's hosts. Often dubbed FEV-Pre1, this effect results in a new evolutionary path, creating a creature that is both old AND new but different all the same.
No one knows why this happens, or how often it occurs in the world.
However, it has had a remarkable effect on keeping one such pre-war creature alive. One thought too have been long extinct, unable to cope with the drastic world-wide changes.
The Dorse, Equus Poeciloaglao (the Horse of Varied Beauty), is one such result of this backwards evolutionary path caused by FEV-Pre1, with its ancestor, the modern horse which was long thought to be extinct, being subjected to the FEV virus, but instead of evolving forwards, like many others, it evolved backwards instead. The effects of this transformation, resulted in what is now known as the Dorse, a small ungulate (hoofed) animal seemingly out of prehistory whose appearance greatly resembles that of the Mesohippus, an extinct genus of early horse, which lived some 40 to 30 million years ago from the Middle Eocene all the way to the Early Oligocene. 
Sharing many traits with it's ancestor, the Dorse truly does appear as a creature out of prehistory, even though it was never like that originally. One interesting biological adaptation is that this equid stands in, as a new-fully tridactyl horse in the evolutionary record, with the third digit of it's foot. being longer and larger than its second and fourth digits; completely ridding itself of the typical hoof of modern-day equids. Along with this new foot-gear, it also shares traits of both horse and pre-horse ancestors, it stands roughly 34 inches (86 cm) at the withers at around 4 feet long and weighs anywhere between 75-80 pounds, while larger individuals can weigh roughly 100-200. With wide-set eyes that sit high atop its long, horse-like skull, the Dorse is also equipped with slightly longer legs than its predecessors, and is endowed with what is a relatively large brain, about the same size, proportionate to its bulk, as that of modern horses.
Changes in physical appearance, as well as changes in breeding, gestation and how often females can produce foals compared to their modern-day ancestors have also helped to aid this animal through its long and treacherous journey to the future, but the most important change that spared the horse from a slow painful trek to extinction involves literally looking these gifted horses in the mouth.
Something interesting to note about the dorse, that sets it apart from its pre-war ancestor, is the arrangement of its teeth, which allows the Dorse to have a wide-ranging diet, best suitable for life in the wastes, where food is scarce and you must eat anything you can get, with the arrangement of the teeth suggesting a diet akin almost too that of a bear, making them opportunistic omnivores who tend to consume more plant than animal matter allowing them to eat anything from leaves, roots, and berries to insects, carrion, fresh meat, and fish, while also having the digestive systems and teeth adapted to such a diet. If the animal stayed on the path of a modern-day horse, limitations in their diet would have affected their survival in a big way, as modern day horses are not as suitable for eating a wide-variety of food if the cause to need too would arise compared to the diet their ancestors once had. 
As it stands the FEV-Pre1 has benefited the modern-day horse in a huge way, but they are not the only ones.  The discovery of the Dorse has not only proven that the FEV virus is capable of more remarkable things then once thought, but the discovery of this particular anomalous result of the FEV has opened the door to the notion that it’s not only horse’s that could have been affected by it. Many species since the war have been thought to be extinct, unable to cope with the vast ecological and environmental changes that came from the result of the great war. However, the appearance of the Dorse, opens the gate to the idea that many other species might also be affected by FEV-Pre1 in the same way. And given how long and magnificent certain animals treks were to the future, the possibilities are endless as to how far back in evolution FEV-Pre1 can take you