Lewis Short
burdo | burdus, i (noun M) : (collat. form , , Acron. ad Hor. C. 3, 27, 7)
* A mule (the offspring of a horse and she-ass, while mulus is the offspring of an ass and a mare; v. Isid. Orig. 12, 1, 61), esp. used for carrying litters, Dig. 32, 49: onus duorum burdonum,Vulg. 4 Reg. 5, 17.
Charlton T. Lewis, Charles Short, A Latin Dictionary