Both of these templates specify that
Special Attacks: A zombie retains none of the base creature’s special attacks.
Special Qualities: A zombie loses most special qualities of the base creature. It retains any extraordinary special qualities that improve its melee or ranged attacks.
I cant seem to find clear distinctions of what is a special quality. However special attacks seem clear enough. Any ability that provides an alternate to the full attack or standard attack seems to fit this.
Special qualities retained is less clear. Abilities like pounce, grab, and rake seem like they should stick around, however the Skeletal Megaraptor also seems to retain racial skill bonuses, as well as scent. A skeletal owlbear however, does not.
What about poisons? Poisons seem like special qualities that improve a melee or ranged attack. However it doesnt make as much sense for a skeleton to have poison. Zombie maybe. However they are magic, so anything could go.
So is there any clear definition as to what stays after the creature has been undead-ified? I can see arguments for various features, and some examples of certain things like racial skill bonuses. However it still seems uncertain to me and many forums and things I have read seem mixed on what stays and doesnt.