animal is ? animals are ?

是这样的，我们老师要我们仿写insects are our good friends这篇文章，于是我想写动物是我们的好朋友，我指的不是所有的动物，那么，各位哥哥姐姐们，我应该用题目中的哪一个呢？
sorry , 我只是一名小学生，所以问得可能有点幼稚，还是请哥哥姐姐们告诉我一下，最好能说一下理由。我不是让大家帮我写，我只是问一下，动物是我们的好朋友这句话中，动物应该用单数还是复数形式呢？谢谢

( )horse is ( )useful animal. My favorite animal is ( ) which animal is your favorite?同意句 this animal is monkey.这样说对吗？还是this animal is a monkey.? which animal do you like best?=which is( )( )favourite animal? The hourse is a useful animal. Horses are useful animal都是马是有用的动物， my favorite animal is dolphin中animal能用复数形式吗 ( )is the largest animal in the world,which breathe with( ) The 考拉 is _____tree---climbing animal which lived in _____Australia. a whale is more an animal than a fish.翻译