The pug is not the only dog with a pug nose, but because of his name he stands out from the rest. In veterinary terms, he's a brachycephalic dog. This term refers to a number of breeds with a broad, flat-shaped skull and "squished" nose. This physical characteristic makes it difficult for these dogs to breathe because they have a very short nasal passage.
Apart from the pug, the oriental Pekingese and shih tzu are both brachycephalic. The King Charles spaniel has a pug nose and French bulldogs and Boston terriers share this characteristic. The largest brachycephalic breed is the pugnacious-looking English bulldog. Images of English bulldog skull formation show that in earlier times it had a longer nasal passage, but this has been bred out during the 20th century. In 2009, the UK Kennel Club announced changes to the English bulldog breed standard in order to stop breeders producing dogs with exaggerated characteristics that create health problems.