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Articles before a common noun:

The cow--- a particular cow.

A cow--- any cow.

Cows.---all of them.

But while writing an essay on cow, we usually get to see:

The cow is a very useful domestic animal.

In my opinion I should be:

Cows are very useful domestic animals.

And the title should be Cows instead of The cow.

Please help.

Thank you.

This is a special usage of the word "the". It is listed in the MacMillan Dictionary under definition #2 as follows,

"used before a singular noun when making a general statement about people or things of a particular type"

They give two examples:

The average university student is not very interested in politics.

People have come to depend on the car as their only means of
transport.

Used in this way, "the + singular noun" means basically the same thing as that same noun pluralized:

the cow = cows

the English garden = English gardens

the computer = computers

This construction is very common in titles and introductory sentences within paragraphs and chapters because it has a certain air of scholarliness about it that the simple plural form just doesn't convey.

The cow is a common way of referring to a species, as well as just a specific individual cow.

Cows refers to all of them.

It's slightly more formal to write of "the cow" than "cows", and is normally preferred in an essay.

A cow is a useful domestic animal.

is semantically equivalent to:

Cows are useful domestic animals.
AND
The cow is a useful domestic animal.

Usually, the use with the is more academic. In everyday language, we tend to use the plural.

So, in fact, any of them could be used in a title.

It's a difference between the abstract and the concrete. Adding "the" before a noun this way changes the meaning, from actual instances of the concept to the concept itself.

Other answers have stated correctly that semantically the two are equivalent but one just "sounds" more formal or more academic. That's true, but I believe the reason is that "the cow" refers to the concept of cows, whereas "cows" refers more directly to, well, actual cows; and speaking at the conceptual level is common in scholarly writing.