The proliferation of mobile devices with increased capabilities and the widespread penetration of wireless networks have spurred a convergence of mobile and commodity computing systems. In this respect, botnets have also found their way to the mobile ecosystem, where they exploit characteristics of phones (e.g. linked to user accounts, repositories of private and sensitive information) to have a profound impact on operations and user privacy. We report here on an in-depth taxonomy of mobile botnet features, architectures and use cases in order to highlight their particularities and accordingly instigate systematic research efforts to alleviate related security concerns.