We're just now catching wind of the fact that, back in March, the federal government quietly changed the definition of "service animal" under the Americans With Disabilities Act to include only dogs and (get this) "small horses" who provide approved types of assistance for those with physical disabilities. San Francisco has for some time embraced a much broader definition of "service animal," including those that provide emotional and psychological support, and the city says it will continue to follow state guidelines in that regard.
SF Weekly notably took on the topic of SF's ultra-lenient policy on service animals back in 2009, which included mentions of an emotionally therapeutic iguana, and some hamsters that helped alleviate one woman's suffering over not being able to conceive children. Most people in town probably know someone who has abused the system and either gotten a placard for their poodle (a spokesperson for Animal Care & Control admits they "don’t use discretion" in giving these things out), or just printed a fake one off the internet so they can bring their dog on Muni trains. That practice is not likely to stop, however the new rule changes mean that private business owners, like restaurants and grocery stores, can begin denying animals on premises based on the new federal rules. Expect some fights over this one, here in pet-adoring, anything-goes SF.