Is a Mentos ONE Mento?

It depends

Explanation:

When you buy a package of Mentos, and want to offer one to someone else, do you say, "Want a Mento?" or "Want a Mentos?"?

Is Mentos plural, or is it one of those words like "moose" which is the same in the plural as it is in the singular?

Mentos are multiple mentos (plural). A mento is just one singular mento.