A Clump Of Stumps

Twilight Sparkle's horn glowed feverishly as she concentrated on Pinkie Pie's leg - or what was left of it.

"Ow, ow, owie, owie," whimpered Pinkie, clutching above the wound with her forelegs.

The wound began to seal shut.

Twilight realized, with growing horror, that this was the wrong spell. It was one of the only first aid spells she knew, but it was for sealing papercuts and scraped knees. It was sealing off Pinkie's leg below the knee, above where her ankle would be if it weren't in a timberwolf's belly.

Pinkie would be left with a perfectly healthy and smooth stump, with no scar or pain. Twilight glowered at the swiftly sealing stump, wishing to high heaven she knew how to make a growing spell work instead.

Pinkie held a funeral for her leg that weekend. It was more of a wake, except she couldn't present the corpse for a proper wakening.

A prosthetist from Manehattan took a plaster cast, and within the week, she was hopping all around Ponyville like before. Still, it was odd to take off her hoof at night, and to wonder what her little hoofsie was doing right now.

The night of the full moon, she took off her hoof with a scraping noise. "Huh, that's strange," she thought, and felt the bottom of her stump.

Rock.

It felt like rock, and it felt longer than it had before, slightly. Pinkie turned the light back on, and walked to the mirror. It was definitely rock, and it was growing...