This interesting creature is made by a glitch. Creeper was actualy a pig with difrent shape. I will show you some pictures... 
You see here pig and creeper compared side by side. As you can see pig and creeper have same body parts solved in difrent way.



Then,Notch decited to change texture of that creature in to green-camo thing color with black eyes.
Thats how creeper was made. I hope i answered your question. If you like my first creation on this site you can give me diamond or favourite it :D.That woud be amazing>!.