Season 1:

Long ago, on the planet Eternia, there was a group of simple-minded snakes. They acted as any other snake would. The snakes would travel through the ground, and they would hunt their prey. One day, the snakes came upon a large statue of a large snake with three heads. The snakes rested near the statue, but what none of them realized was that Serpos was glowing and then, so were all of them...

...

The snakes woke up and realized they have developed limbs like the humans. They all look at each other. One was the most unique. He was the most humanoid and had the ability to turn into a five-headed snake creature. He knew this meant Serpos has chosen him to lead the others in greatness. He became known as Hiss.

To Be Continued

This is Non-cannon to the Ultimate Advanceverse.