|The following is based on Fallout Shelter and some details might contradict canon.|
Skeleton - The most popular Halloween costume of 2077 - an American circa 2078”— In-game description
The skeleton costume is a black bodysuit with white bones. It provides the wearer an added +2 Strength and +3 Luck.
- Found by dwellers exploring the wasteland
- Found in lunchboxes
If successful attempt disassembly of outfits you can get junk:
Skeleton costume (1)
Free space in the storage room
Teddy bear (1)
The outfit was added with the 1.2 update, two weeks before the start of the winter 2015 Holiday season, with events like Halloween and Christmas coming up.