Sure-footed pack animals
(Other definitions for asses that I've seen before include "Animals of horse family" , "Hardy and sure-footed animals" , "Jennets" , "Fools; animals" , "not the most sensible types" .)
(Other definitions for yaks that I've seen before include "Wild oxen of Tibet" , "Rabbits" , "animals" , "Tibetan oxen" .)