User-centered design for animals
Bird feeders aren't really for birds, they're for people to watch birds.
Cats are just as happy with their new scratching post as they are with the box it came in.
Dog clothes are chosen by people for aesthetic reasons, not because they decided to dress like a delicious hot dog.
Plenty of animal products serve important functions, but many must also appeal to people. This challenges typical approaches to User-Centered Design while blending with Animal-Centered Design.
Sensory perception for animals and humans should be met. What smells good to your dog might not smell good to you in your living room.
Species-specific needs must be met and natural behaviors encouraged. A cat toy probably won't work for a catfish.
Understanding different animals' cognitive abilities make feeders intuitive to birds but not squirrels.
Ergonomically, a ball should be comfortable in a hand or mouth.
Animal product design is a large and intricate multidisciplinary field which this post only touches on in some areas. What I find interesting is that many animal products' successes are found in their unique ability to create interactions and experiences between animals and people. It's a complex dual focus on the user and buyer.
Unfortunately for our friends, the focus can't solely be on them when they don't have pockets to pay from.