Hindu mythology is not static. It has changed over history. 

In the Rig Veda (1,500 BCE), we are introduced to Indra. He is a powerful celestial being who rides chariots and who defeats enemies like Vritra, Pani and Vala to secure stolen cows and release water. We don’t hear stories of devas and asuras fighting. The idea of devas and asuras fighting comes in later Brahmana literature (1,000 BCE). In Brahmana literature, we also have the story of Manu saving a small fish from a big fish, and of Prajapati taking the form of a giant turtle and a giant boar, to help the earth float on flood waters.