"Where No Man Has Gone Before" was not the first episode that aired, the first one that aired was "The Man Trap", and neither of them are anything like a pilot, since they just plunge straight into the events of the episode without particularly introducing the characters
Well it's all fine to hope that a pilot will introduce the characters and set up the beginning of the story, but a pilot is made to illustrate the potential for a series before going into production whether it introduces characters or not. "Where No Man Has Gone Before" was made for this purpose, and here in TV land, that's called a pilot.