骑马 is a verb-object compound (离合词): 骑 means 'to ride' and 马 means 'horse'. When adding aspect particles (了, 过), time expressions, or frequency, the two characters separate: the verb takes the particle or measure word, and the object follows. For example, 骑过马 ('have ridden a horse'), 骑了三个小时的马 ('rode a horse for three hours'). You cannot say 骑马了 as a single unit in many contexts — splitting is required.