Somehow I read the title as "Severed pony heads as decorations" at first, LOL!
A lot of G4 ponies had head / body mismatch from day one, due to their heads being made of a different type of plastic than their bodies. The first year of the line was probably the worst for it (which is too bad because they had some awesome pony designs . . . but yeah, very prevalent mismatches.)
And yeah, it can also worsen over time. I have one early Twilight where her head got much paler. She now looks like she's seen a ghost, ha ha.