I have great news, I found The Secret Life of Pets concept art! These look great, and we can see early concept art for Max and other pets. Enjoy!
UPDATE: I found more concept art!http://ludogavillet.blogspot.fi/search/label/the%20secret%20life%20of%20pets
And from here you can find more, but it's wierd that some characters are named wrong. And there's a dog that named Iris, must be deleted character.