The Goodie Bag is a random drop from any Monster during Halloween (October 21 - November 10). It must be noted that it will only drop during Halloween. Meteor Heads and monsters spawned from Statues will not drop any Goodie Bags.
When opened, the Goodie Bag has a 25% chance to give you 4-40 Rotten Eggs or 2.88% chance for one of the items below:
It is opened by right clicking on it in the players inventory.
If the player receives costume pieces, which are worth 60 silver each, then it is worth more when sold after opening the bag, as opposed to the one gold from selling the goodie bag; however, there is a chance you will get rotten eggs, which have no value.
Much like presents during Christmas, Goodie Bags can be farmed year round by setting the computer date to Halloween. (Oct. 24th thru November 10th.)