Lucoa has a dragon form in the manga. It was introduced AFTER the anime was made. KyoAni needed a placeholder Lucoa in dragon form for the opening, which is what you can see on this image.
I personally think they really could've spent a little more time on this placeholder. I wouldn't call it "bad", but it really doesn't fit her at all, and Tohru even says your human form depends on your dragon form and vice-versa. You'd expect a massive dragon who looks serious but not threatening (which is exactly what it is in the manga), not this.
Or those fan-made or official dragon forms of Quetzalcoatl and Fafnir?
Those are the official designs... at least in chibi form... but Lucoa definitely has a different color scheme in the manga because when she finally debuts her dragon form, she shows up as white and light greys which means she probably has a brighter color palette
