To go in with stuff earlier, gotta just have, all of the Fearsome Critters
Rather than the Thunderbird being a phoenix variant, I'd recommend poking at its myths some more. They're not just 'large bird', they're protectors. In addition, I can't find the source on it, but at least one area had myths of thunderbirds specifically shedding their skin to live among humans. Said people oculd then later don those skins and go back to their previous forms in times when their true power would be required.
They would really, really hate Yuan-Ti though. Constant in the Thunderbird myths is the hatred of snakes and beings from the underworld