Ministry of Internal Affairs and Communications researched Japanese population by Oriental Zodiac (animal symbols) in 2013. According to it, the people born in the year of the Ox are the biggest group among 12 zodiacs. The year of the horse is the smallest group.
Table: Japanese Population by Oriental Zodiac, 2013
Population (x 10,000)
Source is Ministry of Internal Affairs and Communications 「午年生まれ」と「新成人」の人口