This is a beer float. Not a root beer float, but a float made with porter, a style of dark beer historically related to stout.
As you can see, in addition to beer it includes ice cream, chocolate sauce, and whipped cream. At the first sip, you notice the absence of the sweetness normally provided by root beer. After that, you settle down to a pretty tasty treat.
Want to try one? Head to the Shamrock Brewing Company pub on West 3rd Street in Pueblo, Colorado, a few miles south of Route 50.