Stubborn sorts are individuals who are determined to stick to their beliefs and opinions, no matter what. The term can be used to describe people, animals, or even objects. In this crossword, the solutions MULES (5 letters), ASSES (5 letters), and DIEHARDS (8 letters) all refer to stubborn sorts. MULES and ASSES are both animals known for their stubbornness, while diehards are people who are especially determined and unyielding.