Excerpt from Fact Sheet: Highlights of the Final Rule to Amend the Department of Justice's Regulation Implementing Title III of the ADA http://www.ada.gov/regs2010/factsheets/title3_factsheet.html
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."