He’s a peacock and you never see his tail. Do you know how big peacock tails are? If it’s hidden in his pants, he must have a hard time walking XDD

That's 'cause in the manga and anime you never really see his tail, guess Paru found it too awkward to get a good composition going with it. Dom essentially appears during the first chapters when Paru was still figuring out things design wise (like really compare her drawings from the first chapters to the ones in the mid-late chapters and it's a world of difference in a good way).

That said, while I was searching for reference images of Dom, I did come upon at least one of him with a proper tail. It was fantastic!