Peachy Pie is a minor, mentioned only character.
During the WarEdit
Peachy was in the Old Olneigh hospital where she was exposed to tainted, Impelled Metamorphosis Potion (I.M.P). The tainted mixture turned Peachy into the first Hospital Horror. Her fate afterwards is unknown.
After the WarEdit
It is possible that she escaped and impregnated the other ponies in the hospital, like the Chimera did with the Stable Dwellers in Stable 24, explaining the high number of Hospital Horrors in Old Olneigh's hospital. She could've also survived into the modern day and the rest of the hospitals occupants became exposed to the contaminated I.M.P.