If you're talking about EQ1 -- it's something that can currently happen, it's just not very used yet because it's still a leeetle bit under construction (well, and also unpublicized.)
Putting {{EQ|type=Undead}} at the bottom of a bestiary entry will display a purdy little box that says the mob is undead, and will also add it to category:Undead.
You can track all currently tagged undead mobs
here -- but at the moment, there are a grand total of three.