The Krampus Sack is a Backpack item. It is an extremely rare drop from Krampus, with only a 1% chance to drop. It has 14 inventory slots, 6 more than a Backpack and 2 more than a Piggyback. It cannot be placed in the player's inventory or in Chests, and it can't be lit on fire.
Even though the importance of only two extra slots may be minimal, the Krampus Sack doesn't slow the player down like the Piggyback does, making it a much better alternative. However, with a lowly 1% drop rate and a significant time investment to spawn a Krampus, it should be said that during any given game it's possible that the player might spend more time trying to get the item than would be saved by not walking slower with the Piggyback. It may be better to ultimately treat the Krampus Sack as an unexpected bonus and/or something to only strive for in one's spare time.
The quickest way to spawn Krampus is by killing a Glommer, so players should take advantage of every full moon cycle to retrieve the Glommer's Flower and kill the Glommer in an area where any spawned Krampus will quickly die. Neutral mobs will be aggressive towards Krampus, so a Pig or Bunnyman village would be an ideal location to kill a Glommer. Tooth Traps work as well, but Krampus might steal the traps instead of setting them off.