Can I add a status icon for thunderbolt ethernet to the menu bar? (Mavericks)
Did you try this from: https://discussions.apple.com/thread/5598818

I quote


  Accordingly, I went into /System/Library/CoreServices/MenuExtras/ then double clicked on WWAN.menu for cable broadband. I think for DSL you'll need to click on PPPoE.menu.


Give it a try, otherwise you probably need third party apps like iStatsMenu