I'm assuming you're thinking of Season 3|Episode 21 "Family Guy Viewer Mail #1" but he also appeared as a kid in a Mini Wheats spoof in the Season 2|Episode 11, "A Picture's Worth a Thousand Bucks", and as a baby in Season 6|Episode 6 "Padre de Familia", and in various stages of growing up in Season 3|Episode 5 "And the Weiner Is"
I Am Peter, Hear Me Roar
Peterotica
Season 2|Episode 1 Peter, Peter, Caviar Eater
No Meals On Wheels
This never happens in an episode of Family Guy.
Peter's parrot on Family Guy, Adrien Beaky, appeared in Long John Peter
Peterotica
Family Gay is the Family Guy episode that Peter gets a horse in.
No Meals on Wheels
Family Guy Petoria Episode.
The 255th episode aired on Family Guy was "Peter's Sister"
Season 9|Episode 10 Friends of Peter G
Season 2|Episode 1 Peter, Peter, Caviar Eater
No Meals On Wheels
family guy viewer mail
it is Season 8 Episode 16 April in Quahog
Trading Places
Season 7 Episode 16