Anthropomorphism - the attribution of human traits and emotions to non-human entities, including animals - has existed probably since the dawn of time. One ancient example is Aesop's Fables, in which stories such as The Hare and The Tortoise have been recounted to numerous generations. In the 19th century, Lewis Carroll's Alice in Wonderland was and continues to delight audiences to this day - it includes a talking caterpillar, mice, flowers, and late hares in full human dress.
In the same era Alice came out, the Victorian Era (1837-1901), there was a trend of photographs or paintings of animals in various anthropomorphic poses, doing human things and wearing human clothing. An example of a Victorian Christmas card:
Victorian ad on a trading card (as popular as Instagram and Facebook are today):
Other artwork go far in the realism department - this art installation is actually taxidermy:
So you see, dressing up your dog in human clothes is not so bizarre after all, and definitely not a new obsession. Anthropomorphism has arguably always been with us and will continue to be popular with a segment of the population, as evidenced by the success of artist William Wegman in the late 20th century. Today, anthropomorphic animals are ubiquitous not only in books and movies (Disney, anyone?), but in artwork as well.