Although he is rather smart indeed for an animal, Pluto is not an anthropomorphic character. A stray puppy, Pluto was a frequent playmate of the young Mickey, Donald and Goofy when they were young; however, as the three kids grew up, they left the pup to grow on his own, too, and when Mickey and Pluto met again one fine day in 1930, Mickey did not recognize the large stray dog he had become, and was surprised that the dog seemed to follow him around and imitate him in every way. At Minnie Mouse's request, he eventually officially adopted him and gave him a proper name, Pluto.
Behind the scenes
Although a hound resembling Pluto was first featured in September of 1930 in the Mickey Mouse short The Chain Gang, the character himself wouldn't appear until October in The Picnic, where he is properly introduced as Mickey and Minnie's pet. It should be noted that the story Pluto the Pup, which was the introduction of Pluto in the comics, was only printed in 1931; however, it can be assumed that it actually took place in 1930, before The Picnic.