Polymorph (hex, etc.) is bugging out with the graveyard mod.
For example, in this video Malygos had been polymorphed but was not actually killed. The sheep was then killed, but the graveyard mod was putting Malygos in the graveyard list instead of sheep. This also happens with shapechange spells such as Hex.
The sheep (frog, etc.) is only added to the graveyard list if the sheep is resurrected and killed.