Skeleton keys are absolutely real! I use to collect them as a child. Some of the older homes that were built nearly a century ago will still have interior doors with the old ward locks. Imagine my surprise when several pieces in my collection had unlocked the old doors in my grandfather's house. Sometimes you'll also find old desks, chests and other interior furniture that lock with skeleton keys.
The formal term skeleton key refers to any key of various designs and types that has been fabricated, simplified or stripped to its "bare bones" to create a master key that can open multiple locks. The informal term popularly refers to the distinct, skeletal-like appearance of the antiquated lever-lock keys, which is what most people will think of when they hear the term.
Skeleton keys are still in limited production for use in many things both practical and artistic. They can also be found in abundance through various online markets and hardware stores.