What type of horizontal line is this in Microsoft Word?

Does anyone know what type of line it is and how to create another?

It's a horizontal line.

The "normal" way (to insert one) is to go to the "Borders" button in the "Paragraph" group and choose "Horizontal Line."

You can also use "Auto Format"

On a blank line, type the 3 characters in a row and then Enter. A line appears across the page.

If you need to remove one, simply Backspace or Delete as normal.

Source 4 Creative Ways to Insert Horizontal Lines in Microsoft Word

And how to change once inserted:

In my opinion this is a Paragraph border, which is applied from Home > Paragraph > Bottom Border. Then you can customize the way you want.

It is a border just like a text box border.

Source: http://answers.microsoft.com/en-us/msoffice/forum/msoffice_word-mso_other/variable-length-horizontal-line/bfec04cc-930f-4b0f-bb05-5d40d5e7074d