"The question was, which came first, hybrid mules or pure species horses?"
The answer is obvious. It is impossible to have a 'hybrid mule' until you have 'pure species horses'.
"It's a question of context: a practical matter, not one of aesthetics: ie mares & jacks are chosen for breeding mules, b/c otherwise (due to chromosomal differences between sp. E. asinus and sp. E. caballus) you'd have a hybrid hinny, instead of a hybrid mule"
The chromosome difference applies whichever direction the hybrid occurrs. The reason 'mares & jacks are chosen for breeding mules' is because mares produce larger offspring than do donkeys, and mules are used for work so size matters. That's why hinnys are so rare. They're useless for anything.
"--Horses and donkeys are different species, with different numbers of chromosomes".
Chromosome difference is not necessarily a probalem because Przewalski's horses and domestic horses have different chromosome numbers yet produce fertile offspring. In fact that has become a problem for maintaining the purity of Przewalski's horses.
"The colloquialism, 'superior', is commonly used by breeders or agriculturalists when comparing two species being considered for hybridization. It refers to the more biologically complex of the two compared"
I have had a great deal to do with agriculture and animal and plant hybridization, and I have never heard such a reference. Hybrids are utilized to obtain the most useful characteristics of each breed.
"All of my elaborate 'horse talk' is for the purpose of an easily understandable ~example. The same basic principles of hybrid theory apply to virtually any two mammalian species close enough taxonomically to breed".
What about hybrid vigour?