Those creatures pass through different stages of evolution that allows them to evolve and learn new movements. Some of them, at first, can't even move but when time goes by, they evolve and walk around the virtual world with no problem.
The total of creatures that can live the world at the same time is 25. You'll be able to see information about the evolution of the creature in the screen.
Finally, if you come back and the screensaver is turned off, when it will be launched again, the evolution will continue from the moment you last saw it.