Learn English – Word for a person with a negative attitude

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What is the person called who always thinks negative? E.g.

Mr. X always thinks negative.

Mr. X has negative attitude.

What we can use either? Is there any particular word for this?

The word is pessimist, a person who expects the worst.

Mr X is a pessimist.

Contrast with optimist.

It is often said that while optimists are happier, pessimists are more often correct.

and

Optimists are said to live longer than pessimists, though a pessimist might say those extra few years are bound to be miserable.