Snoopy said he was an “only dog” in 1959 but, theoretically, he could be the oldest and his brothers and sisters were born after that strip. Yes, I know this would be “headcanon” of the most convoluted sort, not the least because dogs are typically born in multiples, though Beagles can have from 1 to 10 puppies at one time.
I haven’t seen this strip in decades (it was in our local paper and I had some of the paperback collections in the 1970s!), so I’m glad I found it again after all these years — and the “Wizard of Id Classics” page, which is closer to what I remember.
I always think “Oh no, the annual football joke groan” but then I read it and laugh. I really should know by now to never underestimate Schulz.