But,to my surprise,when he gets home,he saw the animal waiti

正确的是：but to my surprise,when he got home,he saw the animal waiting at the door。或者是：but to my surprise,when he gets home,he sees the animal waiting at the door。错在时态不一致,这种句型一般用一般过去时的比较多,但关键还是看你的题目是什么时态的。