Interestingly, Peanuts started with a focus on Shermy and Violet as the 'straight men' and young(er) Charlie Brown as the comic upstart. Snoopy shows up fairly soon, but he doesn't even seem to be CB's pet for the first while.
It's fascinating to see Lucy, Linus, Schroeder and Sally grow from tots or babies to the developed characters we know today.
https://kotaku.com/how-snoopy-killed-peanuts-1724269473
about how Peanuts lost it's edge once the "cute" popular dog was introduced, whereas prior it used to be more subversive, philosophical/theoretical with darker material.
1. Snoopy becoming Flanderized, as in the "Happiness is a warm puppy" stuff from the 1960s.
2. Introduction of Woodstock the bird. That meant Snoopy and Woodstock went off and had their own adventures which didn't involve the human gang at all.
I also wonder whether Schulz participated in any recreational drugs in the 1960s. I don't meant to be disrespectful at all, but some of the stuff he drew was pretty wild.
There's a set of strips where Charlie Brown sees the moon as a baseball (and later, Alfred E. Neuman's head), another where Snoopy dreams of Charlie Brown flying him like a kite and him crashing to the ground in pieces, and a horror-movie-like series where Linus's blanket attacks Lucy. All very strange.