An octopus prop bet specifically requires a player to secure the ball in the end zone on both the touchdown and the conversion. An octopus bet in football can cover a quarterback, but they would need to actually run the ball in themselves before also running in the conversion.
The bookmakers set their odds to make a profit whatever the outcome, the odds are based on money expected to be placed and statistical probability. Many OLBG member create their own betting odds before the bookies have priced up an event, it's a very worthwhile exercise and should become part of your betting life.