Every villager can be a GCN starter, but some more likely than others. The reasoning for this is because there's several species in game which have several villagers that belong to. However, some may only have 2 or 3 that belong to it. Like Octavian, who I believe is the only octopus in game. So they'll have a low probability of turning up. Which is probably going to make him harder to get, since he's the only octopus you can get. Which means you have a 1 out of however many villagers there are in game chance of getting him. (Or an octopus villager in general) I hope this makes sense.