Reserves, or second XIs have existed since the start of organised football. Clubs had enough members to field a number of sides, the first XI were usually older men and would represent the club at important friendlies and cup-ties with the remainder making up the second and sometimes third XIs. The latter sides would normally compete against likewise elevens and junior clubs.
There were numerous competitions for these sides since the earliest days. for eample the Edinburgh 2nd XI Cup first played in the 1870s and the Scottish 2nd XI Cup which existed from 1882 to 1998. A plethora of cups were introduced during the 1880s but the advent of professionalism a decade later put pressure on club finances and during the 1890s these tournaments gradually fell away.
Leagues specifically for reserve XIs didn't really take off until after the Second World War. There were a number of attempts at creating dedicated reserve leagues, but these generally only lasted a season or so before non-League clubs were being admitted. In general, those clubs that did run second XIs pre-1915 placed them in one of the various regional leagues that were in existence such as the Northern League (eg Dundee) and Scottish Combination (eg Rangers) or else in one of the junior leagues.