Exclusive Formables are specific types of formables that can't be made by a formable of a starting nation. For example, if you form the 
Soviet Union, you can't form the 
Russian Empire. These types of formables exist mainly because two formables of the same starting nation (
Russia in this case) have similar borders in-game.
Examples include: Russian Empire/Soviet Union, Kushan Empire/Hephthalite Empire, Austrian Empire/Austro-Hungarian Empire, Songhai Empire/Mali Empire.
IMPORTANT: If you did form one of the formables, the decision to form the corresponding formable will still appear, however you will not be able to interact with it due to the criteria "Is NOT <formable>:" being false. Thus, clicking the decision will do nothing, even if you have all the cities you need for it.
Do not suggest this as a bug in the discord or wiki, as it's an intended feature.