You sneak amputee unicorn into mirror pond grotto

>amputee unicorn shushes you as you sneak her into the mirror pond grotto
>no need to wake the creatures nearby in the thickest of trees
>you unstrap your backpack and lower her gently to the ground
>she uses her forelegs to wrap the straps around her shoulders like suspenders
>with a sort of tripod hopping shuffle, using the backpack's reinforced bottom as a shoe for her legless rear half, she heads to the mirror pool's edge
>she recites the phrase
>another her climbs from the mirror pool, rear half intact
>she helps her up and out
"Hey, you're me! Do you need surgery? My Cutie Mark says that's what I'm good at!"
"First, we need a full surgical team. Repeat after me..."
>there are now four of her
>they're going to "practice" a limb transplant
>one is the doctor and one is the nurse
>one is the transplantee and one is yours
>you're the one with the gun, you very American Anon
>send them back to the pool the hard way
>just in case
>tendons and muscles down there have atrophied
>the surgical team confers and decides on a hemicorporectomy
>the transplantee is curious about the idea, and wants to watch
>you hold the mirror so she can
>the team closes the seams on yours
>the transplantee put the discarded pelvis with only stubs on herself
>she's really proud
>yours wakes up
>she trots around, grinning
>you've never seen her so happy
>she sends the surgeon and nurse back into the pond
>the transplantee seems to get it
>she tries to run, then hop, away
>yours hits her with the banishing spell
>her top half turns to mist and floats toward the pool
>her bottom half remains
>yours falls to the ground in horror
>her guts are on the floor
>her pilfered rear half zips back into the pool as a cloud of magic
>you run to her
>hold her hoof
>she shudders in your grasp
>her eyes glaze over
>your tears mingle with her blood
>the end


Inspired by these posts:
>mirror pool clones of amputees have all their limbs intact because they're based on the pony's residual self-image

What if they have BIID?

The only way it'll produce a copy with fewer than four legs is if the original was born that way or had their limb removed early enough that they can't remember having it.