Family Guy has by far the worst continuitys and retcons of any cartoon, they always contradict themselves with no explanations.
In the season 1 episode Brian: Portrait of a Dog we see a flashback of how Peter meet Brian, he was homeless as an adult and Peter decided to bring him home.https://www.youtube.com/watch?v=kxxenECgrfo
However, in season 7, it is shown that the Griffins has owned Brian ever since he was a puppy, which completely contradicts what we saw in the first season.https://www.youtube.com/watch?v=Q5Sz8XLlx1g
Seriously, how the fuck can you make this kind of mistake? This is basic Family Guy canon, yet the shit writers still managed to fuck it up.
They do this shit all the time,they fucked up and said that Chris is adopted despite Lois giving birth to him, Meg being born as a boy, but Peter cutting her dick off, despite we CLEARLY saw she was always a girl, and Quagmire's age being wrong. Fuck this shit.