Hi, I've recently come to the Dresden Files series and as an avid roleplayer I'm looking forward to the release of the game.
On topic, is a Tibetan Mastiff the same breed as a Chinese Foo Dog (http://puppydogweb.com/caninebreeds/chinesefoo.htm)?
I think with the recent references in the books, it's likely that Mouse is a Foo Dog (Temple Guardian).
There's some more details in the following links.
WARNING: the following links may contain SPOILER information
http://dresdenfiles.wikidot.com/mouse
http://en.wikipedia.org/wiki/List_of_The_Dresden_Files_characters#Mouse