Before there was a draconic demon, there was simply a dragon. The demon king's draconic origins are something he values extremely highly and he gives incredibly high favor to other dragons like himself.

Although his bipedal body tends to be more practical, with his vast demonic powers, he crafted himself a form similar to what he once was but larger, more powerful, and with all his demonic additions. When the king truly wishes to go all out and pay absolutely no mind to discretion or collateral damage, he'll assume the form a unstoppable dragon and all who provoke his ire will regret being born.

Yet another beauutiful piece by biglovealicia
Felnir at his core is a dragon so he should have the ability to have the form of one. You'll be seeing this just as much as his other look from now on. Also will probably add some armor and accessories to this form too at some later point.