Beta Testers are penguins who have been on Club Penguin for a long time, since October 24, 2005.
Dark Falkner: This is sort of a reply to the last answer. Yes beta testers for Club Penguin are old but that doesn't mean EVERY old penguin is a beta tester! A beta tester is a person (err penguin?) who tests a game for bugs and/or glitches before it is officially released to the public.