Road to the Multiverse - S8-E1
Other mistake: In every universe - except for the double-headed universe - Brian and Stewie travel to, they (seem to) replace the version of themselves that universe already has of them, but then in the Dog universe they have a separate version of themselves that they continuously interact with.
Answer: Lacey Chabert was the actress who originally voiced Meg in early episodes. She left of her own accord due to being in school and other acting work; Seth MacFarlane has stated there was no tension with her leaving, but it makes for a handy joke for the show.
Jon Sandys ★