On his sofurry.com site he is named White-Fenrrill with 2 L. On furaffinity.net he is WhiteFenrril with 1 L.

He links to his FA from sofurry as "White Fenrril" and this leads me to believe he prefers the name with a space in it, thus:

whitefenrril -> White_Fenrril
White_Fenrrill -> White_Fenrril
whitefenrrill -> White_Fenrril

Well, from what I've seen spaces are usually not used in artist tags when it's an alias (and not their RL name). I assume it's to match the account name as accurately as possible -- for example, maxblackrabbit or mushicrosshairs (as opposed to dennis_carrigan or bethany_sellers, where there is no account under that name to match spelling/spacing/symbols of).

Either way, considering that you can put spaces in a SoFurry name but they used a hyphen instead, that'd indicate the accurate name would be with a hyphen.

Interesting. I didn't know about spaces allowed there.
Anyway, true. Most usernames are without spaces, and real names (Why do you insist on using them instead of usernames is another topic) use underscores.

So I only propose this to keep it sane as he uses 1 L and 2 Ls.

whitefenrrill -> whitefenrril