In Old Testament times, a man’s wealth was measured by the size of his flocks and herds. Job was a rich man by these standards. Female donkeys are mentioned in this passage because they were more valuable than male donkeys. They milk produced by females was a valuable food substance.
JOB 1:3 – His [Job’s] substance also was seven thousand sheep, and three thousand camels, and five hundred yoke of oxen, and five hundred she asses [female donkeys]