Mule

The infertile offspring of a male donkey and female horse, mules weigh an average of 20d10+800 pounds. They are generally more tractable than donkeys and more intelligent than horses. A average mule can carry 200 pounds (or 20% of its body weight) for up to 16 miles before needing to be rested. Mules can also be taken underground, and will traverse stairs, making them of great value when traversing the great dark passages beneath the surface world. Mules live 1d6+24 years on average. The author suggests using these modified statistics for mules, and valuing them as 12 gp.