Excuse me if I missed the answer... I've searched the forums, but all the FAQs seem to be about basic download problems.
I downloaded a user-created animal from here. (Example: Arctic Fox by Penguinman)
I installed it into the downloads folder (within All Users). It shows up on the download list (from the main menu) and I can adopt it in the game.
But when I filter the animals in the game to show downloads only, I only see 3 BF/MS animals. If I filter to tundra, the fox shows up, and it is there in the full list as well.
Did I miss something obvious, or does the download filter just show official creations?