Lore-wise, it might be similar to the Ziz, but its design is not. The design draws influence from Japanese dragons. The ONLY similarity to the Ziz is drawn from its relations to Kyogre and Groudon. It's a weak argument to say a Pokemon is based on something just because it's related to another Pokemon.
Fire is from the idea that you can burn something and from the ashes it will be reborn, like in nature when volcanoes erupt, destroy islands with volcanic ash and fertilize the ground for reorigination. This is because Reshiram is the nature Pokemon and it's supposed to also be part of the yin-yang idea; fire can be destructive as well as life-bringing. The same is true for electricity, which is why Zekrom has that typing. It's also appropriate because fire and electricity are both sources of light.
Pokemon did a good job designing these two, tbh
Except, Reshiram has feathers -___-