I had to climb to my father's shoulders to see the horses, and then realized that they hadn't been saddled.

saddled 是动词 saddle 的过去分词形式，在句中表示被动。saddle 本义是"马鞍"，作为动词时表示"给马装上马鞍"。saddled 在这里表示"（马匹）被装上马鞍"的状态。