按照经典的规则是应该加es的，因为袋鼠也是有生命的。但是现在两种说法都有。如果你用谷歌搜索一下英文网站，你会发现用kagaroos的比用kagarooes的要多得多。下面这句话来自维基百科关于袋鼠的介绍，就用了s，而没用es：Larger kangaroos have adapted much better to changes wrought to the Australian landscape by humans and though many of their smaller cousins are endangered, they are plentiful.
所以说现在通行的说法是加s。这种现象是强势规则的推广的一个例子，就像好多过去式、过去分词原本有特殊变化的动词，越来越多地变得和规则动词一样，过去式过去分词加ed就可以了。
