Beast Quest: Kaymon the Gorgon Hound by Adam Blade

Kaymon was as large as a bull and her black fur was tangled and caked with filth. She gave off a smell, too - a foul, choking stench that made Tom reel. As the Beast pounded forwards, Tom saw the evil light in her eyes and the terrible strength of her shoulders and flanks. He also noticed something else - the Beast wore a thick collar of twisted gold around her neck, and set deep into the precious metal was a huge white jewel.