A cast of dozens is hard enough to manage in a story -- obviously a novel and a good thick one, too, to have a place to put them all, but you do have to remember to keep track of them and put them in logical locations.
But for some reason, the fox is particularly prone to vanishing from the text. Not because it's not a character -- chatters up a storm on occasion. Knows lots of useful stuff, and does useful stuff.
But when it's not in the front of the story, I keep forgetting to keep track of the fox.
Perhaps it's that it's helping the other character, not advancing its own plot. The mirror does that too, but I did stick the mirror in a room to be consulted, which does help localize it.