Horospherical stacks and stacky coloured fans

We introduce a combinatorial theory of horospherical stacks which is motivated by the work of Geraschenko and Satriano on toric stacks. A horospherical stack corresponds to a combinatorial object called a stacky coloured fan. We give many concrete examples, including a class of easy-to-draw examples called coloured fantastacks. The main results in this paper are combinatorial descriptions of horospherical stacks, the morphisms between them, their decolourations, and their good moduli spaces.