The rider was slowly dismounting his horse before the finish line.

dismounting 是动词 dismount 的现在分词形式。dismount 由前缀 dis-（表示"离开、分离"）和 mount（"骑上、登上"）组成，字面意思是"从骑乘状态下来"，即"下马"或"下车"的动作。