Mary Lou is Sam's donkey that he uses to help him make is onion products and Sam tells Mrs. Collingwood to ''Rub it on her husband's head every night and he will sprout a mane like Mary Lou's.'' and when Sam was rowing away on his boat also called Mary Lou Sam had Mary Lou with him and Trout Walker's motorboat caught up with Sam's and Trout shot Mary Lou and Sam. Sam claims that Mary Lou is over 100 years old, though Katherine Barlow does not agree saying ''How would you know, Sam? You're not a day over 24.'' And later in the movie when Zero runs away from Camp Green Lake he finds Sam's boat called Mary Lou then when Stanley finds Zero, Zero asks him what Mary Lou is, pronouncing it as "Mm-ar-yuh. Luh-oh-oo". Stanley corrects him by saying ''It says Mary Lou.''