The Wii U GamePad went through multiple revisions during development.
Pre-2011
The Pre-2011 GamePad was one of the first pieces of hardware conceptualized for the Wii U, consisting of a screen connected to two Wii Remotes.
2011
The E3 2011 GamePad was similar to the released version, but with many design differences, most notably the presence of circle pads instead of analog sticks. However, this was not the only 2011 prototype GamePad; there was also the CAT-DEV v2 GamePad, which was an even earlier version of this GamePad which was black and rectangular, being somewhat similar to the pre-dev tool v3 Wii Remote in appearance.
2012
By E3 2012, the GamePad was finalized.