So I am working on creating a ponaga (a pony who is also a naga) character for a story I am working on. Any suggestions?

They're generally referred to as snakeponies, nagapons, or just lamias. Ponaga is not a used term to describe them.

The only truly general words of advice I can have is that lamias tend to be forces in their own right. Whether they have image issues from their tails, or are confidant and in control, or are tsundere or whatever else they might be, one does not ignore the fact that the tails are huge, heavy, and entirely made of coiled muscle. When dealing with a snakepony, characters should be fearful, unnerved, or respectful because of the danger of lightning fast coils and pounces, or they should soon have a lesson in humility to demonstrate why it's a bad idea to dismiss those aspects of a snakepony out of hand. Any other details really depend on the archetype, personality, and role of the lamia in your story.