Yes, it can. There are a few causes why it can be denied: You have no correct documentation. You should check carefully the main requirements for an ESA Pet Letter: the official letterhead of a licensed health professional, the license number, the issue, and the expiration date. The airline can deny your dog if they consider your pet is not an emotional support animal or may be dangerous for having a flight. They can deny if your dog is too large. Also, they can prohibit animals younger than four months. If you are an Illinois citizen, from the 2020 year, you have specific rules about emotional support animal letters for landlords. You can check them here.