You can see the horse's main more clearly in the traditional character: 馬
马上 Mǎshàng looks as if it might mean 'on horse back' but actually means 'immediately'.
你是美国人吗？ Nǐ shi Měiguórén ma? 'Are you American?'
妈妈骂马吗？ Māma mà mǎ ma? 'Is mother scolding the horse?'
(骂 Mà means 'scold'.)