Here's another example where it's not so much the amount of sea slug based Pokemon that's surprising (there are only two evolutionary lines of sea slug-based Pokemon), but rather it's surprising that a whopping six designs based on sea slugs were introduced in a single generation in Diamond and Pearl.
Again, someone on the Pokemon design team must have really been inspired by sea slugs, because not only did they base legendary Pokemon Manaphy on the clione, but in a first-ever for the series, allowed a legendary to breed and produce offspring. Breeding a Manaphy produces another sea slug, Phione, who's basically a poor man's Manaphy but still officially maintains legendary status according to the Pokemon Company.
Alsosomewhatof afirst, in the same generation Pokemon designers granted two distinctdesigns to a single Pokemon – both Shellos and Gastrodon have strikingly different appearances depending on whether they're from East or West Sinnoh. It's definitely not unheard of for the same species of Pokemon to have different forms, but Shellos and Gastrodon are unique because the difference in theirtwo forms is purely cosmetic and based on geography.