This is sort of snow idea I've had recently, though now that I look at it I'm not too sure it is terribly original.

My thought is what if in an alt universe (or something else entirely) instead of banishing monsters to Tartarus, the Moon or turning them to stone, they were Turner into a metaphysical form and placed inside of other ponies who would serve as "vessels/containers" for these beings.

It isn't like possessing exactly as the pony is still in control, however they might exhibit traits of the being the contain. For example a mare serving as the vessel for Tirek might possess similar horns.

Just thought I would throw this out and see what you all thought.

That sounds really cool, the should also be able to use some of the beings powers should they choose

6701728 Well, that isn't quite what I had in mind. I kind of feel that might contradict the whole "containment" aspect, but that is just me.

6701980
I feel this would be like some beings would be under partial containment like Discord while others like Tirek whose powers are too dangerous will be under total containment, so a Discord container pony could use some of his power while the Tirek container pony couldnot due to Tirek's magic and what it does as well as the threat he is. In otherwords those in the Partial Containment are in the may be reformed later basket while the Total Containment are in the never to be reformed basket.