Twain probably had several good reasons for using a frog as the title character of his great short story. One point he was trying to make in the story was that "thish-yer Smiley" would bet on absolutely anything, including any animal contest imaginable. Unlike horses, dogs or other larger animals that might be likely to compete in a race, a contest between frogs could be held in a small, confined area, not unlike a poker game. Frogs were also in abundance in the nearby swamp, so they were a ready commodity. But the choice of a frog probably better illustrates the comic absurdity of Smiley's addiction, not to mention that its small size better allows for the twist of the con--pouring bird-shot into Dan'l,'s belly to weight it down.