Why can't we say that "Horses are a useful animal." treats horses as a class or set of objects?

（1）Computers are important research tools.

（2）Computers are an important research tool.

We can say that the first sentence treats computers as discrete objects.

The second sentence treats computers as a class or set of objects.

（3）Horses are useful animals.

（4）Horses are a useful animal.

We can say that the third sentence treats horses as discrete objects.

Why can't we say the fourth sentence treats horses as a class or set of objects?

I think sentence #4 does treat horses as the class of all horses, and that's exactly the problem with it.

It is possible for a class of objects to be a tool, if those objects collectively form a resource that is useful for accomplishing some task.

But it is not possible for a class of objects to be an animal, which is by definition a single living being. So sentence #4 is not grammatically wrong, it's simply nonsensical.

There is nothing inherently wrong with using horses as singular to refer to the class of all horses, so long as you use it in a context where that makes sense. You could certainly say something like:

Horses were an important method of transportation in the 19th century.