Eohippus, the smallest theorized horse ancestor is actually thought to be more like a rock badger (Hyracotherium) – it may be a stretch to claim it as a horse ancestor. In fact, there are other questionable assumptions about the timing of the evolving horses:
“In north-eastern Oregon, the three-toed Neohipparion and one-toed Pliohippuswere found in the same layer. This indicates that they were living at the same time, and thus provides no evidence that one evolved from the other.11,12”
Did the theoretical evolving horses have differing amounts of rib bones?
Living horses have a wide variety of sizes. “Horses vary in other ways too. Modern horses can have 17, 18 or 19 pairs of ribs.”
But what about the number of toes, don’t we see a progression from multiple toes into one toe in modern horses?
In fact, “the genetic information coding for extra toes is present, but is switched off in most modern horses. Sometimes a horse is born today where the genes are switched on, and certainly many fossil horses also had the genes switched on.”
“…Also, three-toed horses are known today. O.C. Marsh himself noted that some horses in the American southwest had three toes of almost equal size, ‘thus corresponding to the feet of the extinct Protohippus.’13”
Read more here: https://creation.com/the-non-evolution-of-the-horse