The "Sack Man" is one of the seemingly infinite alter-egoes of the legendary Bogeyman and much like the "Monster Under The Bed" is a specific form the creature is said to take - however unlike most versions of the Bogeyman the dreaded "Sack Man" may have roots in a very real and tragic tale of old.
The "Sack Man" is envisioned as an impossibly ugly and skinny old man carrying a large sack who wanders the land searching for "naughty" children - whom he is said to capture and stuff in his bag before spiriting them away to a grisly fate (many tales say that the "Sack Man" eats his young victims).
The "Sack Man" was likely inspired by a very real man who, during the 16th and 17th centuries, was in charge of collecting orphan babies in order to take them to the orphanages: he would put them in a huge bag or in wicker baskets, and carry them all through the province collecting more children.
Most of the children usually died before reaching the orphanage due to the lack of care and the obviously insalubrious conditions in which he transported them: the lingering memory of this figure may well of inspired the bogeyman as envisioned in modern lore.
The "Sack Man" is also known in his native Spain as El Hombre Del Saco and is prominent in Latin countries such as Brazil, Portugal and Spanish America.