I believe the purpose of the horns in the monsters is to make them look scary for work and thus appear charming to the monster sexes. Assuming that the monsters mate sexually, the selection of the horn numbers will depend on the whether more numbers of it make one look beautiful or whether it incurs any work related advantage. A small monster colony has a large amount of monsters that have an abnormal amount of horns. This indicates that the abnormal number of horns are preferred over fewer horns as they carry a certain advantage of having a scarier look.
