The article states that there is not a reason that a St Bernard was used, but this is from the SK Library
In the spring of 1977 Stephen took his motorcycle to a mechanic who lived outside of Bridgton, Maine, "in the middle of nowhere". "I took the bike out there, and I just barely made it. And this huge Saint Bernard came out of the barn, growling. Then this guy came out and, I mean, he was Joe Camber-he looked almost like one of those guys out of Deliverance. And I was retreating, and wishing that I was not on my motorcycle, when the guy said, 'Don't worry. He don't bite.' And so I reached out to pet him, and the dog started to go for me. And the guy walked over and said, 'Down Gonzo,' or whatever the dog's name was and gave him this huge whack on the rump, and the dog yelped and sat down. The guy said, 'Gonzo never done that before. I guess he don't like your face.' And that became the central situation of the book, mixed with those old "Movies of the Week," the made-for-television movies that they used to have on ABC. I thought to myself, what if you could have a situation that was an extension of one scene. It would be the ultimate TV movie. There would be one set, there would be one room. You'd never even have to change the camera angle. So there was one very small place, and it became Donna's Pinto--and everything just flowed from that situation--the big dog and the Pinto."