In Mark Twain's short story, "The Celebrated Jumping Frog of Calaveras County," the author's use of the vernacular with Wheeler and Smiley might serve one of two purposes.
On one hand, it might indicate that the men are from the same part of the country, and perhaps used to telling long tales and chatting it up around a drink, or at a table of card players. One gets a mental image of old timers trading stories back and forth, almost in the oral tradition, in the back woods, where stories would be recounted aloud but never written down.
On the other hand, especially in light of the fantastic details of the some of Smiley's animals, that the man who sent the speaker looking for such "Smiley" had an ulterior motive, perhaps to play a trick on the speaker, who ends up cornered and nailed to the spot by Wheeler who has many stories to tell his "new" audience. Dozing on a stool when the speaker enters the bar-room, Wheeler is well-rested and quickly begins his epic string of tales.
The animals have unusual characteristics, and Twain's use of personification of the animals makes the story so much fuller and more entertaining.
For example, Wheeler describes Smiley's mare—it's hard to imagine this old nag moving any of her four legs as Wheeler describes:
...but always at the fag-end of the race she'd get excited and desperate-like, and come cavorting and straddling up, and scattering her legs around limber, sometimes in the air, and sometimes out to one side amongst the fences...
Wheeler also tells the speaker about Andrew Jackson, Smiley's dog that could beat any other dog by clamping onto one of the dog's back legs and refusing to ever leg go. Unfortunately, Jackson is put up against a dog that had no back legs: the story seems simply impossible:
Smiley always come out winner on that pup, till he harnessed a dog once that didn't have no hind legs, because they'd been sawed off in a circular saw...
Even more amazing his Twain's description of this almost-human dog who loses, is "offended," "blames" Smiley (examples of personification), and goes off to die:
...[Andrew Jackson] come to make a snatch for his pet holt, he saw in a minute how he'd been imposed on, and how the other dog had him in the door, so to speak, and he 'peared surprised, and then he looked sorter discouraged-like, and didn't try no more to win the fight, and so he got shucked out bad. He give Smiley a look, as much as to say his heart was broke, and it was his fault, for putting up a dog that hadn't no hind legs for him to take holt of, which was his main dependence in a fight, and then he limped off a piece and laid down and died.
The use of the vernacular by Wheeler and Smiley adds to the charm of the piece. More importantly, however, as the speech is exactly the same when Wheeler is speaking or Smiley, I would venture to guess, that as the speaker supposes in his introduction, there is no Smiley at all, and the tall tales Wheeler is telling are of his own devising. It is probably a rare occasion that they are aired out at all (with the townsfolk knowing better than to get "sucked in") except when a stranger comes to town who doesn't know any better—and ends up sitting and listening while Wheeler spins some real "yarns."