Normally, the verb agrees with the subject in number, so

This region has snakes.

Those regions have snakes.

Countries are normally singular:

Jamaica has snakes.

even if grammatically they look plural:

The Seychelles has snakes.

The United States has snakes.

What might be confusing you is that collective nouns (nouns denoting a collection of people) sometimes take a plural verb when the sense is that your are talking about what the people do or are, rather than what the group as a whole is:

Manchester United has/have scored a goal.

The committee is/are worried.

And this can even apply to names of countries when it means a the national team in some competition:

Jamaica has/have scored a goal.

Using plural in these cases is more common in British than American English.