While this isn't a baseball card picturing a dog, it sort of fits into the theme of my recent three-parter.
It's a picture I picked up a couple of years back. I'm going to have to go from memory, here, because I can't lay hands on the actual photo right now, with its attached caption, but I believe it was taking in spring training 1951.
The picture shows Brooklyn Dodgers shortstop Pee Wee Reese with his toy Manchester terrier (I don't believe the dog was named in the photo). Looking on are infielder Bobby Morgan (left) and outfielder George Shuba.
It takes a big man to own a tiny dog.