Source: Tafsir al-Ahlam al-Kabir (Ibn Sirin)
The mule in a dream is a sterile man or a long journey. Riding it indicates a long life. A white mule indicates a religious man, and a black one indicates a rich man.
Source: Ta'tir al-Anam fi Ta'bir al-Manam (Al-Nabulsi)
The mule indicates a child of adultery (from zina). It may also indicate a man of no origin.
Source: Al-Isharat fi Ilm al-Ibarat (Ibn Shahin)
The mule is interpreted as travel. Whoever sees himself riding it will travel far.