hm. there are 10 mono-white dragons, and 25 other dragons with white, with most dragons being red, but there are no exclusively red+white dragons
not just legendary, just none at all
maybe the Boros Legion has a strict ban on recruiting dragons because every time they send one to boot camp at least a dozen valuable drill instructors get eaten
I know there's dragons on Ravnica because, y'know, Wile E. Coyote is reincarnated as one there. Plus there's ones printed in Ravnica sets with Gruul, Izzet, and Rakdos watermarks.
There's also dragons in Jeskai, Naya, and Mardu colors, and 5 color/colorless, but no Boros.
(Also no Golgari, Orzhov, or Simic dragons either.)
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