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If I remember correctly, for the starters there was also the recurring theme of the three getting their secondary type at different stages:
>Mantis being part bug since the beginning
>thorny devil part ground since the first evo
>snail/slug part rock with the last evo
That's why the design of the dragon slug has its shell only in the last evolution, and that's why I support of it. A redesign might be useful though, even though there was one that was better than the actual one in the pic.