Which ingredient is unique to the Dark and Stormy cocktail?

In the Dark and Stormy cocktail, ginger beer is a defining ingredient that adds a distinct spicy and tangy flavor profile to the drink. The cocktail traditionally consists of dark rum and ginger beer, which gives it a refreshing yet robust character. The ginger beer contributes effervescence and sweetness, balancing the rich notes of the dark rum. This combination is essential to the drink, making it unique and easily identifiable.

The other ingredients listed are not typical components of a Dark and Stormy. Tequila is a spirit used in different cocktails but not in this one. Whipped cream is generally associated with dessert cocktails or drinks that require a creamy texture, which does not align with the Dark and Stormy's profile. Peach schnapps, a flavored liqueur, is also unrelated to this cocktail's taste and style. Therefore, ginger beer stands out as the crucial, unique element that defines the Dark and Stormy.