If I say "horses" instead of "hordes", does the meaning get changed?
Yes, the meaning would change if "hordes" was replaced with "horses". "Hordes" is not a synonym for "horses". "Hordes" or "horde" refers to a large group of people. "Hordes" has a somewhat negative connotation, and implies that the existence of the large group of people in unfavorable. Ex: There was a horde of people at the amusement park, so the lines were long for the rides.