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Of course, long before the iPad, real world tablets were already appearing, While you certainly couldn't call it portable, the basic idea of a tablet can be seen in 1961's RAND Tablet, aka Grafacon (for 'Graphic Converter').
The first prototype, which sure enough looks like an iPad, was the brainchild of Alan Kay: the Dynabook. This appeared in 1972.
What's that? These are all just concepts or fiction? What do you think design is? Apple isn't fighting Samsung on patents, they're saying Samsung duplicated their look. Clearly, the tablet look has been around for thousands of years. But, if it's real equipment you want, real hardware we've got. The first tablet that shipped commercially, as far as I can tell, was the GRIDPad in 1989.