If you've already tried to enter a joomla menu at a location other than the main menu, you will be sure that once it is published it appears with vertically arranged entries.
Personally, I've often done it making life impossible because not always the sidebar is the most suitable place for a site's secondary menus and lately I've been re-turned to the site from which I've taken the pictures of this article. In this case the client wanted the user menu to be hosted above the header and topmenu navigation section.
As you can see from the picture, a menu placed above the vertically ordered header is really out of place and as well as uncomplicated and not aestethic.
It takes very little to enable this hidden feature of Joomla 3.x version:
Administrator side, from the "Extensions" menu> "Forms" click on your "Menu" form.
Go to the "Advanced" tab and enter the word "nav-pills" in the box labeled "Suffix menu class" (as in the figure below), save it and the game is done.