Athletes are jungle animals. I say this because they are usually fit, well-fed and athletically built. They are also fast. In addition, a jungle animal is constantly in competition with others for food and resources, or to be leader of the pack.
How about this: Athletes are the mortal gods of Ancient Greece. They were certainly held in high esteem, ... heck, even little kids wonder why various cartoon characters are literally wearing laurels on thier heads. Therefore, instead of complimenting their action as athletes, I thought it would be fun to comment upon their "immortality" through our observance of the Olympic Games throughout history. Of course, if you want it to refer specifically to that very first set of Olympic Games, you could change it to "Athletes are the mortal gods of the very first Olympic Games in Greece."
If I can answer your question with a question, what kind of impression do you wish to give about the athletes in the first Olympic games in Ancient Greece? This of course will determine your answer. Remember, a metaphor is a piece of figurative language in which you compare one thing to another by asserting a direct comparison, without using the words "like" or "as." Thus, for example, if you wished to indicate the speed and grace of the athletes you could say that they were lithe cheetahs speeding across the track. If you wished to point out the way in which they could be compared to machines, you could say that the athletes were finely tuned pistons pumping forward with incredible energy. If you wished to point towards the way that they glided over the ground apparently defying gravity, you could describe them as being a flock of birds swooping forward towards the finish line. At the end of the day, what you need to do is decide what attributes you wish to stress and then find a suitable object to compare them to.