In the book Norse Mythology by Neil Gaiman (2017) both the names Fenrir and Fenris Wolf are used.
Example, from chapter "Ragnarok: The Final Destiny of the Gods":
Fenrir, the great wolf, will free himself from his shackles... Where Fenris Wolf walks, flaming destruction follows.
This distinction in name usage does not appear to be random or unintentional. As in the above quote, both forms of the name are used sometimes in close proximity.
My first thought was that the term "Fenris" in "Fenris Wolf" was being used as a sort of modifier / adjective. But since "Wolf" is capitalized that does not seem correct. More that "Fenris Wolf" is the full name but "Fenrir" is a type of abbreviation?
I didn't count carefully but "Fenrir" appears to be the more common usage.
I'm interested in anything related to this specific book, its sources, related authentic materials, etc.