就你给出的三个句子来说，意思相同，但强调重点略有区别：

A horse is a useful animal. 马是有用的动物。——强调个别

The horse is a useful animal. 马是有用的动物。——强调类别

Horses are useful animal. 马是有用的动物。——强调整体

至于不定冠词、定冠词、复数名词这三种方法表类指时的区别，章振邦教授的《新编英语语法(修订本)上册》有如下归纳：

三种冠词的类指用法比较

1. 语体的制约

如前所述，定冠词 / 不定冠词与单数可数名词搭配，零冠词与复数名词或不可数名词搭配均可表示类别，例如：

The / A saw is a tool for cutting wood.  

Saws are tools for cutting wood.

The / A refrigerator is a machine that keeps food cold.  

Refrigerators are maichines that keep food cold.

The / A horse is a useful animal.  

Horses are useful animals.  

就所指意义来说，三种表示法无甚区别，但定冠词的类指用法受语体限制较大，通常用于正式语体。一般地下定义，通常用“不定冠词+单数名词”或“零冠词+不可数名词”。在口语中，笼统地泛指一类的人或物则多用“零冠词+ 复数名词”。正由于“定冠词+单数名词” 常用于正式语体，所以在科学叙述中把一个类属与另一类属互相区别时，常用这一结构，例如：

Is the tractor or the harvester more urgently needed there?  

He refers to the peasant，not to the worker.  

Which is the more beautiful flower, the rose or the lily?  

The palm tree lives longer than the oak tree.  

2. 词汇意义的制约

用不定冠词作类指通常是指一类中的任何一个，这样，在某些句子中便只能用定冠词，而不能用不定冠词。比如我们可以说：

The tiger is in danger of becoming extinct.  

The telephone was invented in 1876 .  

但不能说:

* A tiger is in danger of becoming extinct.  

* A telephone was invented in 1876.  

3. 上下文的制约

用复数名词作类指可以表示同一类属中的不同种类或个体。因此，有些句子中便只能用“零冠词+复数名词” 的结构，而不能用另外两种结构，例如：

Soldiers wear uniforms.  

Flowers are not sold there.  

Grapes are much cheaper today.  

反之，“零冠词+ 复数名词”和“不定冠词+单数名词”虽然都可用来下定义，如：

A lion is a wild animal.  

Lions are wild animals.  

A musician is a person skilled in music.  

Musicians are persons skilled in music.  

但在下列句子中以用“不定冠词+单数名词”为好：

A triangle has three angles.  

A square has four sides.  

A force is a push or a pull.  

4 ) 位置的制约

用“定冠词+单数名词”作类指，既可用于主语位置，也可用于非主语位置，而“不定冠词+单数名词” 和“零冠词+复数名词”在非主语的位置上往往改变所指意义，试比较:

He has written an article on the modem play.  

He has written an article on a modem play.  

He has written an article on modem plays.  

在上述the modern play，a modem play 和modem plays 中，只有the modem play 是类指，a modem play 和modem plays 都有非确定特指含义。