Seaman, a Dreamcast game that saw some popularity in Japan was later ported to the Playstation 2, and now possibly the Nintendo DS as well. A translation of Yoot Saito's blog (Seaman's creator) has revealed that Seaman DS may be in development, though at this point it's little more than a rumor. For those who don't know, Seaman is a simulation game in which you take care of a sort of freshwater fish, the Seaman. They begin as a sort of parasite, and evolve from that point up until they become a frog-like creature..
Source: GoNintendo