It's dynamic!

X

Y

X originally came up with the idea of Y in 1968. Although Y could be used by adults and children alike, it primarily targeted children. He envisioned it to embody the theories of Seymour Papert. It was one of the earliest instances of the use of GUI. Although a much more sophisticated and highly popular version of Y (Z) has been released since then, X still believes that the Y, as he conceived it, is unrealized as of yet. Identify X,Y and Z.