Mull

Mulls are large, quadruped creatures, mutated with alchemy and magic a few centuries ago to make the perfect pack animal. Many would argue that these experiments turned out less than perfectly, as the creatures they produced are oddly shaped and prone to ill-health, in addition to the ethical concerns of magical experimentation on animals to begin with.
However, there is no argument that the mutations worked. Mulls, at least when not afflicted by one of the issues of health to which they tend to be prone, are rather docile and easy to lead. And while not especially fast (they can nearly keep up with horses at overland travel, but don't have nearly the running speed), they make up for it with sheer endurance and strength. This strength doesn't translate as well to self-defense, as their muscular bulk makes it difficult to swing their front limbs, but mulls are seemingly almost impossible to weigh down. They have been documented to carry over a ton of goods all in one precariously-balanced load for merchants on multiple occasions.
This makes them especially well-suited as pack animals for miners and other professions that need to carry goods that are particularly heavy for their size, as the only known limits to what a mull can carry is how much you can fit on its back before the whole load tips over.

Conservation Status

Mulls don't breed exceptionally well, likely as a result of the magical mutations, which makes their numbers relatively low. They are an exclusively domesticated species, as they would have a hard time fending for themselves in the wilds. The few mull farms that exist tend to be rather well-guarded, as a major hit to any of their populations could spell trouble for the entire species.
There have been some to say intentional mull breeding should be put to a stop and the species allowed to die out, as they aren't natural, healthy, or capable of lives outside of serving other races. However, as mulls seem fairly content as travelling pack animals from what anyone can tell, this idea hasn't gained enough traction to put a stop to their breeders.