Steelhorn Rhinoceros

The steelhorn rhinoceros was bred by the Seiguenorians to serve in the mines. While working in the mines is well-known to be a prestigious and worthwhile occupation, there is more to life than breathing coal and poisonous fumes day in and day out. While Dargoth wishes for his people to be strong and productive, he does not wish for them to be stagnant. He wants them to grow, thrive, and rejoice in his presence. Mining is none of these things, and a whole lot of stagnation.
Seeing this, one enterprising biologist decided he would solve these problems once and for all. If Seiguenorians could do the tasks related to mining, surely another biological being could as well. First he looked to monkeys, due to their biological similarity to the Seiguen, but they proved to be too weak and fleet-minded to serve in the mines. Deciding to find the opposite of weak and fleet-minded, he got lucky with his second choice, the rhinoceros. They could work all day, take a beating, and attack powerfully with their natural weapon. That said, they weren't perfect quite yet.
After years of breeding, magical enhancements, and many, many failures, he eventually created the perfect creature for working in the mines. A rhinoceros with a horn harder than steel, with hide tougher than rocks, specifically so these two things could mine through and resist their competitors. After some live tests and many more experiments, he eventually raised them to be immune to the poisons floating in the mines as well, with a pleasant side-effect being resistance to all other types of poison.
Then he taught them to innately know mage-hand, because picking up after your pets sucks, and is boring.
For the first time, the Seiguen had created a creature better at a job than they were. This rhinoceros breed has the capacity to mine, collect their load, withstand cave-ins, breathe poison, and walk out as though they were still living in the Land of Shadows. Of course, rarer and tougher metals require rarer and tougher tools, but for general mining labor? Nothing surpasses the steelhorn.
There's a sort of simple beauty in a creature that can both work and carry its load.