Thanks for all the replies. Here's the story and I don't know if it makes it easier or more difficult to guess what it is. There are no markings on this thing.
I thought it was a metal file with an old wood handle. Not sure why a "metal" file came to mind except the grooves are fairly fine, not at all course like for removing serious amounts of wood or other material.
The wife's 78 year old aunt had this in the rafters of a 2 car garage. This garage has been rehabilitated several times over but I never knew this story. According to the aunt, this little homestead was build just after the Civil War. What's now the garage was formerly a stable (and 3 or 4 times it's current size) and her grandfather was a veterinarian. The aunt says this has been in the garage / stable "as long as she remembers". She has pictures from the early 1900's with "Grandpa", some horses and a carriage in and around this garage.
So, is it a hoof rasp? Guess it'd be hard to say. One thing for sure, since the handle doesn't show a bunch of wear, it must not have been used much.