My dog feels safe and comfortable sleeping in his cage at night.

his 是代词 he 的所有格形式，表示"他的"，指代前面提到的"my dog"。 cage 是名词，意思是"笼子"，通常是用金属或塑料制成的围栏结构，用于关养动物。 his cage 指"他的笼子"，即属于这只狗的笼子。