Pegleg's Goldfish is Pegleg's somewhat dead, but very alive, goldfish.
Pegleg's Goldfish is a friendly and polite goldfish.
When Roddy was being flushed down into the sewer, Pegleg's Goldfish came swimming up to him and landed right in his hands. The goldfish asked Roddy, "Have you seen my dad?". Then Roddy got startled and threw him away. He is voiced by David Bowers who is also the director of Flushed Away.
The above quote is probably a nod to Finding Nemo, which had been released the previous year.