How to select character model in creation?
The "Skeleton Model" is from the Skeleton Costumes 1 DLC and occupies the underwear slot of custom characters. Check your console's store page for the game to purchase it. From what I can see, it should be $0.99
Do note that the Skeleton Model does have some conflicts with certain clothing items (I couldn't tell you which off-hand, since I don't own it)