I think it had to do alot with where the legion was recruited. To the best of my knowledge the legions during the late republic which were recruited in spain had bulls on their banners. The ones during the era of Augustis had capricorns on them. Although Legio V Alaude had larks on their banner which is in part to to their recruitment in southern Switzerland and their "barbarian" heritage. If this info is wrong, please correct me.