Undead Nightmare Mask
To see this easter egg you must have unlocked the use of Fences.
In the Saint Dennis Fence’s shop, if you look closely you can see the plague mask from the first Red Dead Redemption’s DLC, Undead Nightmare sitting on the top-left shelf behind the store counter.
The mask can be shot and will eventually fall off the shelf if you shoot it at the right angle enough times. It can also take cosmetic gunshot damage, though it won’t destroy the mask at all. It’s advised to save before attempting this on your own.