I haven't purchased any wolves from the flea market but I have picked up quite a few wild wolves, though I assume it works the same way either way. When you get a new wolf it inputs into the first available space and in the order your dens are listed in. So if you have a free space in your first listed den, that's where it will go. If you don't then it goes to the next one. I don't like this system, I would like my nice looking, dressed up den to be the first one I see when I enter my profile, but its kind of tedious sorting every new wolf I get into a different tab.
Is it a huge deal? No, not really. But it is somewhat tedious, so there is probably a better solution. And I would like others ideas as well. I would prefer if we could select an “input den” where new wolves would default to going in to. Perhaps if that den fills up then the new wolf could go to the next available den after the input den (so the input is treated as the “first” den). Then it could loop around.
Maybe this isn't the most elegant solution so I'd love to hear more thoughts on it. And if I made a mistake on something, please let me know.