As always, the ADA is a Federal floor, or minimum standard. Any State can mandate greater civil rights to its residents than the Federal minimum. The Service Animal guidance I linked to notes, for example, that states may permit other species.
Convergence is in the Twin Cities. A quick check of the Guide Horse Foundation leads me to the Minnesota legislature where there's a law passed this year specifically permitting guide horses, relying on the new special provision for miniature horses in ADA 2010 which I missed while composing my previous post. I am not a lawyer, and it seems that there's a delicate dance going on at the Federal level (a guide horse is not a "service animal," and its admission is subject to limitations which are prohibited against service animals) and at the Minnesota level (a guide horse is not a service animal, but Minnesota's Human Rights Act looks favorably on Federal definitions, so we'll pretend they are until someone sues us about it.)
Re: No they can't require service animal documentation
Convergence is in the Twin Cities. A quick check of the Guide Horse Foundation leads me to the Minnesota legislature where there's a law passed this year specifically permitting guide horses, relying on the new special provision for miniature horses in ADA 2010 which I missed while composing my previous post. I am not a lawyer, and it seems that there's a delicate dance going on at the Federal level (a guide horse is not a "service animal," and its admission is subject to limitations which are prohibited against service animals) and at the Minnesota level (a guide horse is not a service animal, but Minnesota's Human Rights Act looks favorably on Federal definitions, so we'll pretend they are until someone sues us about it.)