The pigman was a humanoid entity that was originally going to inhabit villages but was not implemented into the final game. Their skin can be found within the game files, and their zombified counterpart, the Zombie Pigman, which is now known as the zombified piglin, has been a part of the Nether since the Halloween Update. It has the same dimensions as a player and has no coding present within the game that spawns it.
Notch mentioned on April 25, 2011, that he might add pigmen as villages' townspeople. In Beta 1.9 Pre-releases, a different villager mob was introduced. Regular pig models were changed in Beta 1.8 to have a protruding snout. Whether this will be applied to Pigmen in the future is unknown, although possibly unlikely due to the fact stated above of sharing the same shape as other entities.
The Pigman was a mob that was supposed to be added to the game but was scrapped pre-release. You can still find the entity model in the game files. A zombie version of the pigman was added to the game as a neutral mob in the Nether.
A. No, because pigmen were not added to the final build of the game. You can still see it in the game files.
A. No, even with the use of commands, you cannot summon a pigman into the game.