It is increasingly common for scholars and journalists to make claims of horror cinema’s potential to engage with socio-political realities and, in so doing, identify grave social injustices. This article argues that, if one is to make a true assessment of the extent to which horror films might effect social change, one needs to look towards activist communities within which filmmakers are using the genre as part of a broader effort to do precisely that. In so doing, the article theorizes ‘Activist Horror Film’ in relation to a British short film, The Herd; a film cultivated as part of the vegan-feminist protest movement, designed to influence and then alter the behaviour of audiences. The article begins by situating The Herd within the context of scholarship about socially-charged horror films, before moving on to consider the film’s broader activist context and that of its production, the crowd-funding campaign that led to its completion, the film’s content, the movie’s presence at film festivals and online, and its afterlife within circles of vegan/animal welfare activism. One contends that The Herd, as the first Activist Horror Film, is easily distinguished from other socially-aware horror films of the contemporary moment, for the activism of its makers is what drives it, the context that birthed it, and the context within which it continues to be shown.