The joke of the fable depends on two ancient Greek folk beliefs about these creatures: the mighty lion was supposedly afraid of the rooster's cock-a-doodle-doo, and the mighty elephant was supposedly afraid of the gnat. The Greek version of this fable is actually about Prometheus, rather than about Jupiter. You can learn more about Prometheus and his role as a creator in this Wikipedia article.
The Lion and the Elephant
The Lion, for all his size and strength, and his sharp teeth and claws, is a coward in one thing: he can't bear the sound of a cock crowing, and runs away whenever he hears it.
He complained bitterly to Jupiter for making him like that, but Jupiter said it wasn't his fault: he had done the best he could for him, and, considering this was his only failing, he ought to be well content.
The Lion, however, wouldn't be comforted, and was so ashamed of his timidity that he wished he might die.
In this state of mind, he met the Elephant and had a talk with him. He noticed that the great beast cocked up his ears all the time, as if he were listening for something, and he asked him why he did so.
Just then a gnat came humming by, and the Elephant said, "Do you see that wretched little buzzing insect? I'm terribly afraid of its getting into my ear: if it once gets in, I'm dead and done for."
The Lion's spirits rose at once when he heard this: "For," he said to himself, "if the Elephant, huge as he is, is afraid of a gnat, I needn't be so much ashamed of being afraid of a cock, who is ten thousand times bigger than a gnat."