Just for clarification, the Department of Justice (who writes all this stuff) in Feb 2011, changed the wording, and only animals accepted as a service animal is a dog. Small horses are allowed for guide work. But as of Feb 2011, snakes, monkeys, chickens, etc are no longer classified or protected under the ADA as a service animal.
You can read more about it
here.
#5 reads "Service Animals. The rule defines "service animal" as a dog that has been individually trained to do work or perform tasks for the benefit of an individual with a disability. The rule states that other animals, whether wild or domestic, do not qualify as service animals. Dogs that are not trained to perform tasks that mitigate the effects of a disability, including dogs that are used purely for emotional support, are not service animals. The final rule also clarifies that individuals with mental disabilities who use service animals that are trained to perform a specific task are protected by the ADA. The rule permits the use of trained miniature horses as alternatives to dogs, subject to certain limitations. To allow flexibility in situations where using a horse would not be appropriate, the final rule does not include miniature horses in the definition of "service animal." "