One of the earliest references to the Standard Oil as an “octopus” was in an article that appeared in the Atlantic Monthly, in March 1881. ‘The Story of A Great Monopoly‘: “So closely had the Standard octopus gripped itself about Mr. Vanderbilt [railroads] that even at the outside rates its competitors could not get transportation from him.”
The name stuck and the representation of Standard Oil as an octopus continued on for another three decades.
Lloyd, H.D. (1881), The Atlantic Monthly, http://www.theatlantic.com/doc/188103/monopoly/3 (Accessed: 14th Mar 2009)