Are you sure. Why could it not be a pet? The image in the OP doesn't appear to show a harness (as in the NYTimes article) as is usual for an assistance animal. In my country assistance animals usually wear some high-vis in public too.
A logical explanation doesn't change the analogy. I'm aware that S&M Gimps exist. If I went into an Apple Store where someone was being led around by a gimp, and no-one was paying attention, I'd still think that was unusual.
Not to mention that an aside explaining that helper horses exists detracts from the art of storytelling.
Let's say that I see the horse and I do realize that it's an assistance animal, albeit unusual. If other people doesn't look so surprised I might suspect my ignorance: it seems that all other people know what's going on, and take it as perfectly normal. As I don't want to stand out in the crowd as the only freak who didn't know about this socially accepted habit, I simply play quiet as nothing strange happened.
The same thing could happen in the head of many people and the overall effect would be that of uninterested people not caring about anything.
Group psychology is a very strange and fascinating thing.
(The analogy is with the tale of the http://en.wikipedia.org/wiki/The_Emperors_New_Clothes)
> soon Cox College of Nursing and Health Sciences, where Rose was attending nursing school, refused Richard access, too. Stories started appearing about Rose and her monkey in the newspaper and on TV. “Suddenly,” she told me, “everyone knew I had a mental disorder.”
a monkey on one's back! idiom origin revealed!
[1]: http://www.nytimes.com/2009/01/04/magazine/04Creatures-t.htm...