Then there's always the sapromneme, or "fungal ghost" - a fungal spore which plants itself in a dead body, absorbs the body's nutrients and memories, and rises as a floating mushroom cap dangling tendrils, who now not only has the full memories of the dead host organism but actually thinks it's that person somehow cursed into a strange form. (See Dragon #267, from back in the AD&D 2E days, if you want to update it to a newer version of the game.)
Johnathan