In most cases of successful mammalian hybridization, which female animal is most likely to produce (give birth to) offspring which is both ~viable and reasonably ~fertile: the superior of the two species bred, or the inferior of the two?
Answer: the superior of the two species bred. A female of the inferior species mated, will usually miscarry; or if it should miraculously give live birth somehow, the offspring will almost undoubtedly be sterile (ie unable to reproduce).
Which is most ~primitive taxonomically: the mare horse from which a mule is foaled, or the mule foal?
(I would assume the donkey sire to be the most taxonomically primitive of the three parties involved in procreating a hybrid mule).
Which came (ie existed) ~first: horse(s) or mule(s)? Answer: horses.
Horses (superior taxonomically) didn't "evolve" from the more primitive hybrid mules; nor did Humans from hybrid subspecies of Homo Sapiens sapiens (like Neanderthal or Denisovans, etc.).
The only way to evolve a horse from a mule, would be to backbreed mules with only purebred horses, for many generations -- but regardless, the offspring would still retain some fraction of their male donkey ancestor's DNA in them.
However, their mtDNA would always be that of a horse; because it would be irrational (ie largely biologically unsuccessful, and contrary to the working, practical theory of mammalian hybridization) to breed a mule mare with a stud horse.
~That was "Horse sense", lol~
[NOTE: I forgot to mention the fact that even if a mule mare were bred to a horse stallion -- it would still have the mtDNA of its dam: a horse. Lol.]