You could start a niche website about a specific breed of cat. Another option could be a YouTube channel reviewing dog chew toys. Or you could start an authority site about caring for exotic birds. You could also go offline with your niche business by starting a pet sitting service. Pet owners in some cities are willing to pay up to $100 per day for reliable pet sitters.
In addition, you must know how much you want to earn by joining affiliate programs. Will it be your main source of income or just an additional one? For the first scenario, you should consider promoting products that are expensive and, consequently, that can bring you a good profit. If you plan on the second scenario, you can opt for affiliate programs that pay for sign ups or affiliate programs that pay daily.