The description of the "dragons" tag is quite general, and it would certainly seem to include questions about their anatomy:
"For questions involving dragons - generally large, reptilian, flying creatures famous in folklore and myths."
This being the case, I don't think there's a need for a separate "dragon-anatomy" tag.
We currently only have three questions with this tag, two of which have negative score. Only one of them is also tagged "dragons", so someone using that tag to search for dragon questions wouldn't find the other two.
(I submitted an edit to add that tag to one of them, btw.)
And for someone who's used four other tags when composing their post, it could be difficult to work out which one to use, since the five-tag limit stops them using both.