In Part One of Fahrenheit 451, the Mechanical Hound growls at Montag and this causes him to think that another fireman has adjusted its combination to make it react to him in an angry manner. Over the next few days, Montag's anxiety towards the Hound increases and is exacerbated on day five when a colleague tells him a story about a fireman in Seattle:
"Fireman in Seattle, purposely set a Mechanical Hound to his own chemical complex and let it loose."
In other words, the fireman in Seattle deliberately manipulated the settings on the Mechanical Hound so that it would attack and kill him. This story of suicide is significant because it suggests that Montag is not the only fireman who is unhappy with his life. It also demonstrates the irony at the heart of the story: that book-burning is designed to make people happy but, in fact, creates feelings of emptiness and misery.
Your question is vague. Initially, Beatty sends the Mechanical Hound to Montag's house because Beatty suspects that Montag is hording books. He does this to warn him.
Then in the final part of the book, when Montag is the subject of a televised nation-wide man hunt, another hound is sent after him. This one has been programmed to track Montag's scent and to kill him. He is only able to escape it when he plunges into the river. Finally, when he meets up with the "book men" in the country, one of them gives him something to drink that will change his scent and through the hound of his trail.
He commits suicide by setting a Mechanical hound to his own chemical complex.