User blog comment:B214/Volume 22 spoilers/@comment-3965848-20161204121455/@comment-9595671-20161205230230

I get the feeling Issei chose these outfits for them to where specifically because that's what they're called. *giggles* I mean Issei is the Breast Dragon Emperor after all, I wouldn't put that past him.

I'm pretty sure Tannin's bipedal only in the anime (and later the manga I think) and said to be quadrupel like most dragons in the novels, the only bipedal dragon in the light novels is Grendel.