A pen that is made from the quill of a bird is the type that was used before any of the modern pen types where invented. This means that in a D&D setting these are the type that is most likely to be used. The best feathers for these pens is the primary flight feathers of a goose or swan but what if we look into some of the more fantastic answer that would be available to someone in a D&D setting?
Firsts up what about a Cockatrices feather? What kind of effect might this have if used as a pen? Maybe it allows you to transcribe faster, or possibly all it does is change the inks color to that of stone. I am sure whatever the affect you can think of the outcome would be amusing to spring on the party.
What about an Angels feather, does a quill made of one make it so the user can never write a lie? That would be quite a useful thing for law enforcement. Does a cleric of Good alignment using one become able to write binding contracts with a magic to back it up? What if the feather was not acquired from the angel willingly, does that reverse the affects? A good number of ideas are waiting for this one.
This was just two feathered creatures of some magic significances that I could think of off the top of my head. Even if a quill does nothing magical the parties Magic User may just want to strip a few feathers off that creature they just beat for style. A colorful collection of quill pens to show off all the feathered foes you beat would be an interesting conversation starter to say the least. All you have to do is show them that it is possible and even if they don't go for it a little flavor text for the kings scribe can never hurt.