What badge do I get by creating a new tag? Is it Taxonomist?
Also is there any way to market a tag once created? I believe there is a legit tag missing and I want to create it.
As far as I know, you can't earn a badge for simply creating a new tag. Taxonomist is close, but it does have some restrictions. From the list of badge descriptions (emphasis mine):
Note, however, that you first need 1500 rep to create a new tag on Stack Overflow.
To answer the second part of your question.
The only way to "market a tag" is to ask great questions that need the tag. Great questions gets lots of attention and so does your tag.
If you think that there is a tag missing, and you don't have enough reputation points to create one yourself you have two possibilities:
It just depends on your priority. Credit, or tag availability.