There are visible and invisible disabilities, ones you can see and ones such as mental health issues that you can’t. However, there is a difference between a disability and an impairment. A disability constitutes something that hinders your ability to do an everyday activity, while an impairment is something that can be mitigated by treatment to not hinder as badly.
There are three types of animals in the ADA world: service animals, emotional support animals, and therapy animals. Service animals are the only ones permitted by law to go anywhere humans can go. Service animals are trained to perform a task that directly addresses a person’s disability.
It is not legal to ask for written proof that an animal is a service animal, but there are two questions that are allowed: Is this animal required for treatment of a disability? What service is it trained to perform?
Emotional support animals are not recognized under ADA law, but they are protected under air travel and fair housing laws. Therapy animals are used in professional settings to provide mental health benefits.