If its to replace a noun or reaffirm/remention a noun that was mentioned earlier or will be said later, its only "yun". As another commenter said its short for something different to yung. The removal of "g" is not just to make it sound better. Siya yun, yung basketball player! (Yun = basketball player) Lagi n'yang sinasabi yun. (Yun = yung sinabi) More @Wikipedia
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