Is this one referencing something that I’m missing? Or is the joke at face value, that they’re going off to war, which isn’t a suitable place for a pet?
Well also as kids back in the day (and likely even moreso back in the days of Mr. Larson's childhood), dogs used to be a little more free to roam especially in small towns, this used to kinda be a thing.
Like we'd all be running off to play baseball or do something, and someone's dog would come running out to catch up and go with us, but for X reason (likely we wanted to focus on the task at hand or it wasn't suitable for dogs) the dog was supposed to be staying home.
I think it's really just "wouldn't that be funny if that happened to knights marching to war instead haha" and I agree, it is funny.