It is a scientific fact (probably): A photo of a dog will get more likes than a photo of a human. The 'Pet Economy' is recession-proof. People will cut back on dining out, but they will not cut back on buying treats and toys for Fido.
Custom pet apparel is a massive sub-niche. It falls into two categories: Apparel FOR pets, and apparel ABOUT pets.
Apparel FOR Pets
Dog bandanas are the easiest entry point. They are simple squares of fabric, easy to print, and one size fits most. But the real money is in Matching Sets. A hoodie for the human, and a matching hoodie for the dog. It is ridiculous, it is adorable, and it sells like crazy on Instagram.
Apparel ABOUT Pets
This is where personalization shines. Using the Shirtaki Design Studio, a customer can upload a photo of their dog, remove the background, and add their dog's name. 'The Golden Retriever Club', 'Proud Pitbull Mom', 'Official Cat Servant'.
Design Tip: Offer specific breeds. Don't just do 'Dog'. Do 'French Bulldog', 'Corgi', 'Dachshund'. Owners of these breeds are cult-like in their devotion. If you have a shirt with a cartoon Corgi doing yoga, every Corgi owner who sees it will buy it.
Conclusion
Emotion sells. And nothing triggers emotion quite like the unconditional love of a pet. If you can capture that feeling in a product, you have a winner.