It's a symbol of how good, useful tools we grant to or accept from our government can quickly and easily become tools of oppression. As an example, some would argue that modern US equivalent of this might be the NSA.
Mechanical Hounds were, as Easterly Irk notes, a useful invention, originally. They've found people who might be injured or unconscious. They were good tools. However, as time went on, they grew increasingly oppressive. The government took the technology, what started out as a strong positive influence in the world of Fahrenheit 451, and encouraged its use for illicit purposes, abusing it to suppress free thought.