The mechanical hound is an unthinking robot that seeks to enforce conformity in the populace.
Montag's world in Fahrenheit 451 is characterized by a popular choice against free thinking, reading, and remarkable individuality. This is a world where books are burned because they supposedly create immoral desires in people, causing readers to become ambitious and dissatisfied.
The only way to be satisfied, happy and normal is to submit to the group. The choice of conformity is also a rule. It is the law. And it is a very popular one.
The mechanical hound can be said to represent the society depicted in the novel, which values conformity above all else and insists upon it, creating laws, rules, customs and technology to keep people from acting and living as individuals.