Selecting a good pack animal requires evaluating temperament first and physical conformation second, because a physically ideal animal with poor temperament will be a liability in backcountry conditions where the ability to stay calm under pressure is more important than any specific physical measurement. An animal that panics and bucks off its load on a narrow mountain trail represents a serious safety risk regardless of how correctly conformed it is.
Temperament assessment should include evaluating how the animal responds to being loaded and unloaded, how it handles unfamiliar equipment and sounds, whether it remains calm when separated from companions, and how it behaves when something goes wrong — when it bumps its load on a tree branch or its panniers shift unexpectedly. These are the exact situations a pack animal will encounter in real use.
Physically, a good pack animal needs a broad, well-muscled back of appropriate length that provides a stable platform for the pack saddle, strong hindquarters that provide driving power for uphill work, and sound feet and legs that will hold up to extended miles on rough terrain. Size matters less than soundness — a correctly built 14.3 hand mule will outwork and outlast a 16 hand horse with poor feet and a weak back.