Complex sentence: This specialized over-snow vehicle dominated the snow transportation market until the 1960s when other manufacturers entered the business, by then snowcat was such a common description that it was used to describe all over-snow vehicles, much the same as Kleenex is used to describe all tissues.
Split into simpler sentences: This specialized over-snow vehicle dominated the snow transportation market until the 1960s when other manufacturers entered the business. By then "snowcat" was such a common description that it was used to describe all over-snow vehicles, much the same as Kleenex is used to describe all tissues.