Honestly I forgot as well... It's hard to remember animal's genders ;-;
Gramps introduced Snow as the two adult Fenrir's daughter in the earlier chapters. He said that she was nameless still too, which is why Arito can make her his familiar.
We knew this, but because that chapter was SO LONG ago we forgot she was female because Snow is a unisex name. When they drop the gender on us we tend to forget with animals since it isn't relevant to the plot for us to remember xD
I never know that snow is a girl.. Maybe i forget at some point （/TДT)/