The "Pig Skin?" items from the Bats shows as the normal Pig Skin item on the bottom right slot on the shelf instead of the bat skin itself (Screenshot). Once you pick that one up it becomes a normal pig skin. (Bat skins that you pick up afterwards stack onto that pig skin as pig skins as well.)
Steps to Reproduce
Enter your own house, and put a "Pig Skin?" on the bottom right slot on the shelf.
Enter your own house, and put a "Pig Skin?" on the bottom right slot on the shelf.