What is the significance of the animals Orwell chose to represent the KGB in Animal Farm?

Orwell chose nine dogs to represent the KGB in Animal Farm. It would be more accurate to say the dogs represented the NKVD, Stalin's private security force tasked with intimidating and eliminating his opponents.
The NKVD, and later the KGB, was greatly feared in Russia during Stalin's reign. They were known to execute entire families and led the Great Purge from 1936–38, a campaign of political repression that included rounding up Stalin's enemies for imprisonment, banishment, and executions.
The dogs in Animal Farm are terrifying ("enormous," with "brass-studded collars"), ferocious, brutal, and blindly loyal to Napoleon, the pig who represents Stalin. As soon as a fellow pig, Snowball, finishes making a speech in opposition to Napoleon, the leader whistles, and the dogs run Snowball off the farm—much as NKVD agents made those who spoke against Stalin quickly disappear.
Orwell likely chose dogs to represent Stalin's bodyguards because they are strong, vicious, obedient, and fiercely loyal, willing to give their life to protect their leader—just like agents of the NKVD.
https://www.systemaspetsnaz.com/history-of-the-cheka-ogpu-nkvd-mgb-kgb-fsb