Reptile was hidden in the original Mortal Kombat and thus holds the distinction of being the very first secret character to appear in a versus fighting game. Before battles with an opponent he would make his appearance and jump in the middle of the screen in a Sub-Zero win pose. Above his head would be clues on how to find him, such as "Look to la luna" ("the moon" in Spanish or Italian and hinting The Pit stage), "Alone is how to find me" (denoting that he is only unlockable in single player mode), "Perfection is the key" (directing towards the double Flawless victory), or "Tip eht fo mottob" ("Bottom of The Pit" spelled backward). This concept was then copied for Smoke and Jade in Mortal Kombat II.
He was a merge between the original Sub-Zero and Scorpion characters. Mortal Kombat co-creators John Tobias and Ed Boon admitted in an interview that the "birth" of Reptile was when they took Sub-Zero and Scorpion and fused the two into one and colored him green (yellow + blue = green). They christened this character "Reptile" due to his green color and their reference to lizards. While he wasn't given a lot of character depth at the time, he was worked on more (character wise) and later, in Mortal Kombat II, Reptile was developed into a full character with his own special moves and would be available from the outset.
It is not quite clear how old Reptile is. Estimates run in the tens of thousands of years or even millions. What is known, however, is that the Earthrealm was nearly destroyed by the battle between the gods millions of years ago. Reptile's race of raptors or Saurians, escaped to another realm, Zaterra, where it was then invaded and annexed by Outworld's emperor, Shao Kahn. As was the usual process for Kahn conquering another world, most of the Zaterrans were killed in the ensuing battles, only leaving a handful of people left. Among those, or among those descended from these survivors, was Reptile. Unlike the others of his kind, who were forced into slavery, Reptile was trained to be a servant of both Kahn and Shang Tsung.
Because of this it is unclear exactly what Reptile's place of birth is - although Earthrealm, Zaterra, or Outworld are the three likely choices.