The creepers were based on a failed model of a pig. The length and the height of the pig model were accidentally inverted, resulting in a tall mid section with small legs.
"I don't have any modeling programs to do the models, I just write them in code. And I accidentally made them (the creepers) tall instead of long, so it was like a tall thing with four little feet. And that became the creeper. As opposed to a pig." - Minecraft creator 'Notch'