Looks like one of Jimmy Neutron's incomplete inventions A.K.A a Stub.
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Swimmy is revealed to be Carl's undead goldfish, as revealed in "Beach Party Mummy" and it became alive in the episode later in the episode.

Swimmy shares the same model as the talkative fish in Raise the Oozy Scab and as Cindy in Jimmy Timmy Power Hour 2: When Nerds Collide.