Posting a question late friday night on the start of the Christmas holiday and then repeating it "for the last time" during the evening of the Saturday, less than 24 hours later, is likely to make you disappointed, developers work business hours as everyone else and PDS is not quite big enough to have a "historian on call" for these kind of questions.

Now that I'm here however I'm happy to give an answer to this question even in my free time, as I helped revise this area back when EU4 was originally developed and it's in my opinion one of the more interesting parts of the map.
First the tag in general:
Khorasan is traditionally the name of the eastern part of the greater Persian region. As you can see if you scroll through the historical dates in the game this region was often divided between an eastern and a western pole. The west is more urban and developed with big cities in the many valleys there while the east was less developed (though there are great cities here too) and often saw much more mobile warfare. The traditional capital in this era for Khorasan is Herat in modern Afghanistan but other cities were at times also the capital of Khorasani states. This area was more contested than the Persian heartlands as it's at the border between Persia, whoever controls Afghanistan (after the 1520s this is the Mughals most of the time) and Transoxiana/Mawarannahr (the Timurids at the start, the Shaybanids later on) and often not under quite as strict control as the western part of the region. In times when central authority was weak the east could become more or less independent and rebellions are far from unheard of here.
The region is also described in the second art of war developer diary.
The Khorasan you can see in your 1453 screenshot:
After the death of Ulugh Beg the Timurid Empire was split, one Timurid branch retained control over Samarkand while another continued in Herat and controlled most of the Timurid lands outside of Transoxiana. As it controlled that region the branch in Herat is commonly referred to as Khorasan or the Khorasan Timurids and remained independent until reconquered by Abu Said.
Traditionally (and leaning a lot on the description by the later Timurid prince Babur who founded the Mughal empire) Abu Said is considered the last Timurid prince of quality in this part of the world and after he had reunited the eastern Timurid Empire he went on to attempt to reclaim western Persia. This was the last meaningful attempt to restore Timur's lost empire as after Abu Said eventually failed ("the great disaster of Iraq Ajemi" to Babur) the Timurid lands got divided among the various Timurid descendants again and remained divided (at this point the game could indeed use a few more tags such as Sistan or Balkh).
As the various Timurid splinters then continued to fight each-other (and the princes within these states also continued the proud family tradition of fratricide against eachother) other powers conquered the various Timurid splinter states. In the end the Uzbek Shaybanids and the Safavids got most of the region while one of the last remaining Timurid princes, Babur, went on to conquer India instead (the Mughal Empire).
Wikipedia has a nice table over various Timurid statelets.
I hope that answers your questions, if you have more feel free to post them here and I'll attempt to reply when I see them but I cannot guarantee how quickly that reply will come.
As you seem interested in this region in general I'll take this opportunity to recommend this little book:
http://www.amazon.com/Medieval-Persia-1040-1797-History-Near/dp/0582493242
It's kept very short and really only covers basic political history but it's not a bad way to get introduced to this region IMO

If you're familiar with the game files themselves there are in some cases comments in the country history files of the game that mention what the tags are supposed to be at various starting dates.
Lastly while it is true that there are states in some places that represent various real small countries rather than any specific real state they aren't as many as they used to be prior to v. 1.8 (and Khorasan isn't one of them).
In this particular region Baluchistan is the best example of a state that never existed. That's a region that was controlled by various tribal entities in this era mostly left to their own devises, apart from the area around Quetta and the Bolan pass few of the powers in the region really had an interest to control it either.
Two specific historical states that are represented by Baluchistan at various dates are the Kingdom of Kalat and the domains of the Rinds.