Lewis & Short

mūla, ae (abl. plur. mulabus: data et vehicula cum mulabus ac mulionibus, Capitol. Ver. 5; Tert. Uxor. 2, 8; Prisc. p. 733 P.), f. [mulus], a she-mule; also, in gen., a mule (used instead of horses for drawing carriages or bearing litters): ex asino et equā mula gignitur, Plin. 8, 44, 69, § 171; Juv. 7, 181.
She-mules bear no young, Plin. l. l. § 173. Hence: mulae partus a te prolatus est: res mirabilis propterea, quia non saepe fit, Cic. Div. 2, 22, 49; cf. id. ib. 1, 18, 36: hoc monstrum fetae conparo mulae, Juv. 13, 66.
Hence, prov.: cum mula peperit, when a mule foals, i. e. never, Suet. Galb. 4.