This icon appeared on my Mac Pro with El Capitan.
It cannot be clicked, nor I can see a name.
Any ideas what it might be? I have the same applications on my Mac Mini and never seen the icon.
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I believe it is "Checking for updates" icon for LibreOffice. It goes away whenever you close it, and whenever you click the icon with LibreOffice open, it brings up the "Check for updates" dialog.
I accidentally discovered that while trying to find out what it was!
Downward arrow indicates a download, typically. Are you downloading content without realizing it? Some app that is pulling down data in the background?
Check Activity Monitor (vs your Mini) and see what shows up on your system that is not running on the Mini. Kill it. See if the icon disappears. If it does, you found your culprit.