The difference, by the numbers
They have no ingredient in common: the Harvey Wallbanger has vodka, orange juice and Galliano, the Horse’s Neck Cognac and ginger ale. In parts, a Harvey Wallbanger is 1 vodka, 4 orange and ½ Galliano; a Horse’s Neck is 1 brandy and 3 ginger ale. By volume, the Harvey Wallbanger is 73% mixer and juice, 18% spirit and 9% liqueur; the Horse’s Neck is 75% mixer and juice and 25% spirit. Different bottles, similar balance: that makes them close cousins on the ratio map.
The Harvey Wallbanger is the stronger drink in the glass (10.1% ABV against 9.2%) and holds more alcohol (1.03 standard drinks against 0.92). The Harvey Wallbanger has more calories (about 176 against 132) and more sugar (15.1 g against 11.0 g). Both are built and served in a highball.