Oxen

Oxen are bovines trained as draught animals used for ploughing, transport, hauling cargo, threshing grain by trampling, powering machines for grinding grain, irrigation or other purposes, and drawing carts and wagons.
Ultimate Equipment Guide II
Author Greg Lynch, J. C. Alvarez
Publisher Mongoose Publishing
Publish date 2005
A species of animal renowned for its great strength and fortitude. Though useless as mounts, they make excellent beasts of burden.
Ox: 15 gp