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The first real world tablets crude ancestors were around in 1961.
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').
1989's GRIDPad was the first commercial tablet.
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.
Other tablets, using pen-based computing, soon followed.
The GRIDPad was followed by a slew of other unsuccessful tablets, including—oh the irony—HP's first failed tablet 1992's Compaq Concerto, . It turns out the HP TouchPad had failure in its DNA. Shades of Microsoft Surface, this early hybrid tablet/laptop also had a keyboard.