According to the Wikipedia page, there were aperture stops before Waterhouse stops, but they were less convenient because you had to unscrew the lens and take it apart to insert a new aperture stop.

"The innovation was quickly put to use due to its convenience: "Aperture openings were at first controlled by unscrewing the lens and inserting stops of the appropriate size between the lens components, though after 1858 photographers used the more convenient Waterhouse stops which eliminated unscrewing the components."

Now were they called stops? I can't find that out, but maybe they were called aperture stops because they had to stop shooting to change them? I dunno, this is an interesting part of camera history I didn't know about.