Look, we know the dogs are brutal and vicious, but can you really blame them? Napoleon raises them specifically to be his own little private army: he takes them from their parents as puppies, says that he'll "make himself responsible for their education," and then raises them in isolation from the rest of the farm (3.12). No wonder they become little monsters.
(Click the character infographic to download.)
They're first unleashed on Animal Farm right after Snowball has wowed everyone with his flashy speech. But Snowball doesn't get to bask in his glory for long: Napoleon lets out a whistle, and "there was a terrible baying sound outside, and nine enormous dogs, wearing brass-studded collars came bounding into the barn" (5.14).
From this point on, the dogs are the pigs' bodyguards—they intimidate and threaten the other animals to make sure that the pigs get their way in exchange for treats and cushy beds up at the farmhouse. That's not all: "It was noticed that they wagged their tails to him [Napoleon] in the same way as the other dogs had been used to do to Mr. Jones" (5.15). The narrator is already foreshadowing the novel's end—the pigs are going to be just as bad as the humans.
This one's easy: dogs are symbols for the NKVD, Stalin's freaky and powerful secret police force. During the "purges," the secret police helped Stalin round up everyone who was perceived as a threat and then carried out their exiles or executions. Like the nine dogs, they were the violent force that allowed Stalin to remain in power.
It's just like they say: there are no bad dogs, only bad owners.