@Chr1sLesl3y:
The existence of now extinct species that were larger than their modern extant relatives does not equate to “all animals were bigger better faster smarter and healthier in the past […] without exception”. That is an absurd hasty generalisation. In reality, there were plemty of prehistoric species that are smaller than their modern relatives, especially if you go to the earlier days of the lineage - with horses, there are, for instance, the wide variety of basal horses, Cormohipparion, Equus simplicidens, et cetera. Indeed, there are groups that do not have giant fossil relatives, such as giraffes.
The large American equids were not simply E. ferus but bigger and better. They had different proportions than any modern equids, and thus, they filled a different niche. They were not giant versions of modern horses, they were different, if closely related, animals that were larger, just like moose are not simply roes but bigger. They were giant only that they were a good bit outside of the normal size range of modern horses - they are not that much larger (indeed, the record horse, Sampson/Mammoth, was about the size that E. giganteus is estimated to have been, only a few centimetres less tall but a bit heavier), nor so large that they would overturn our assumptions on how large horses could be. They most certainly were not anywhere large enough to support belief in mythological giants.
The assumption that bigger automatically means “better”, and especially that it means that they were better in aspects unrelated to size. A large size has advantages, the most notable ones being protection from predators (and, if you are a predator, potentially being able to kill larger prey) and better heat retention due to a lower surface-to-volume ratio, but there are also tradeoffs. A large animal requires more energy, and consequently more and/or higher-quality food and a larger area to supply it. It is less agile, especeially if it comes to climbing or navigating confined spaces like caves, crevices or underbrush. It requires special adaptations to bear its own weight. It likely has a lower rate of reproduction and a smaller population site. The improved heat retention can in fact be problematic in hot climates (leading the biogeographic observation known as Bergamann’s Rule, whre related animals tend to be smaller the closer you get to the tropics and larger the closer you get to the poles). Indeed, the rule with island evolution is that, while small species tend to undergo gigantism, large taxa tend towards dwarfism, as their size loses the advantage of protection in the absence of its usual predators, whereas the limited ressources and space impose a major disadvantage. Proboscids in particular provide a good number of examples, such as the Wrangel Island Mammoth, the Flores Stegodon and the various Mediterranean dwarf elephants.
The sauropod in the room is, of course, that, giant or not, are very different from anything alive today. We so much of the fossil record as extinct because these animals do not exist anymore - note that Cuvier’s theory of extinction predates On The Origin of Species by half a century.
Finally, there is no species of donkeys called “Equus Asinus”, let alone a prehistoric giant one. Asinus is the subgenus of Equus that comprises the donkeys, and Equus africanus asinus is the subspecies of the African donkey that is the domestic donkey - obviously, both the subgenus and the subspecies are alive and well and well known to science.