A suggestion works by when you give your idea as the title which you did as “Llamacorn Costume”.
Other than that, you will need to explain the suggestion in depth such as providing more information about it like why would you want this costume to be added to the game and etc. You can also explain on how did you get this idea and how would it benefit Hive and the community. You can check out Sploders topic about suggestions and its format here. 