On the Daltons' Trail (Lucky Luke Adventure Series #19)

Rin Tin Can, possibly the stupidest dog ever, is left in charge of the jail. Obviously, the Daltons escape and steal some horses and guns. Lucky Luke refuses to get involved, but when he witnesses the prison guards' incompetence and Rin Tan Can's remarkable lack of brains, he has no choice but to set out on the Daltons' trail of holdups and burglaries.