In Norse mythology, he is called both Fenrir and Fenrisúlfr. Fenrisúlfr means Fenrir's Wolf, but it seems to be common to translate it as Fenris Wolf.
For example, here's a poem (from Vafþrúðnismál, I believe) that calls him "Fenrir":
"Fjölð ek fór, fjölð ek freistaðak,
fjölð ek of reynda regin:
Hvaðan kemr sól á inn slétta himin,
I will start with question 3, as it is the most straightforward. Gaiman has done a whole lot of elaboration on a passage that is in fact rather short. I will quote the relevant passages from the primary sources, as they are rather short. First Völuspá:
To three there came
from the land
this high and mighty
Aesir to the house,
found they ...
The primary sources
Let us start with what the sources actually say they were given, and by whom. First, from Völuspá:
Mind they not own,
reflection they had not,
no vision nor cover
or colour fine;
mind gave Odinn,
reflection gave Hænir,
vision gave Lódurr
and colour fine.
Then, Snorri, in Gylfaginning, chapter IX:
When the sons of Borr were ...
As Snorri Sturluson's Edda says, in the Skaldskaparmal (pg 164 of my Faulkes translation):
warg, wolf, Geri, watcher and grey beast, Hati, Hrodvitnir and heath-dweller, Freki and forest dweller, Fenrir, leopard, Goti, worthy, noisy, howler, fighter, dusky, dreadful and dark-cheeked.
Further terms can be found on the wikipedia page:
Fenrir (Old Norse/...
I'm going to attempt an answer, based on my knowledge of mythology in general, and Classical mythology in particular, with the caveat that I'd redirect you towards Stack Mythology where the real Norse experts reside.
The separation of qualities among different deities is a staple of most pantheons. Even in fairly binary pantheons such as Zoroastrianism you'...