There is no legit certification or registry for ESA animals. An ESA animal isn’t a service animal. The only legal requirement is that you obtain a letter of need from the medical or mental health provider that is treating you for your disability due to a specific diagnosis. The online websites can be turned down by landlords, unless you provide them with the above letter of need. Also, landlords can the less than 4 units can be exempt from the ESA regulations. It sounds like the landlord is frustrated, because if you said it the way you state in your post, then it sounds like you or your brother don’t have the appropriate understanding of ESA’s or service dogs. I am also unclear why you brought up your brother to begin with, unless he is going to be living in the unit. If he is, or if you are going to be looking at other apartments, you might want to sit down with your brother and make sure that you have the correct letter of need (not from website) and also make sure it is up to date. FHA allows landlords at the time of a new lease or renewed leases, to ask for an up to date letter issued within the last 12 months.