Peachy Pie is a minor, mentioned only character.
During the War
Peachy was in the Old Olneigh hospital where she was exposed to tainted, Impelled Metamorphosis Potion (I.M.P). The tainted mixture turned a pregnant Peachy into the first Hospital Horror, after which it is implied that she was studied and eventually diseccted when a way to reverse the mutation process could not be found.
After the War
Peachy Pie's dissected corpse is viewed by LittlePip and Xenith during their journey through the Abandoned Hospital, with LittlePip noting the tragic fate of what (According to audio-logs) was once an empathetic and self-sacrificing Earth Pony. It is assumed that the other Hospital Horrors found by LittlePip and Xenith were the result of both Peachy Pie's unborn children undergoing the same mutation (Becoming Hospital Horrors themselves) and their unfortunate victims. (The skeletons found with cracked pelvises)