I would tend to agree that this sword (IS it a gurade?
) would be deserving of a high quality hilt and that rhinocerous horn would certainly be in order and not at all unusual, so that's not a problem at all.
I also noticed that the only place that it appears slightly translucent is at the pommel end, which is a seperate piece that appears to be cut cross-grain from the rest of the hilt and even here only at the edges, so it's possible that it was cut from a piece of the horn that had slightly less mass (or possibly from a younger, immature animal whose horn was only partially developed) than the rest for wideness.
When dealing with animal by-products in weaponry, one thing that we tend to forget is that no two are ever exactly alike due to differences in individual animals so a certain amount of variation is not only to be expected, but is actually inevitable.
As to the likelihood of the hilt coming from a local animal, I'd like to interject here that if a material is highly valued enough, no distance is too great or cost too extreme for the very rich or royalty, and since rhinocerous horn is a commonly used material in Islamic weaponry of exceptional quality, it's also a good example as rhinocerous occur nowhere in what is commonly thought of as Islamic territory, with the possible exception of Borneo, and that's just since the acceptance of Indonesia as a unified nation.
Another extremely popular material for hilting, of course, is elephant ivory, this having been so since the days of the earliest cave paintings, not to mention look at the amount of Mammoth ivory that shows up as sword and knife hilts and the regions in which these weapons frequently appear.
While the jury is still out, it appears more and more likely that the extinction of the Wooly Mammoth was indeed largely brought about by overhunting by humans, with the entire tusk probably used as tent supports.
Mike