Arthur Conan Doyle introduces these two vicious animals for a purpose. It is not to suggest that the animals make it impossible for Julia and Helen to leave the house. They seem to be able to travel fairly freely and to meet other people. Helen comes to London on the train. They are not prisoners. Doyle's purpose in inventing the baboon and the cheetah was to eliminate as far as possible the likelihood that an outsider could have been responsible for Julia's death. Everyone in the area knows the grounds are roamed by two dangerous and exotic animals. Doyle could have used a couple of big dogs instead, but the animals help to characterize Dr. Roylott as eccentric, dangerous, and knowledgeable about India. The fact that outsiders are unlikely to venture onto the grounds narrows the possible suspects down to one. Dr. Roylott must be somehow responsible for Julia's death and therefore must be planning the same fate for Helen. Doyle even brings Roylott to Holmes' rooms in London in order to show what a powerful and violent man he is. The reader must surely suspect him of murdering Julia. But how? The question is how he could have killed a girl who was sleeping in a locked room. The presence of the wild animals also adds an element of danger to Sherlock Holmes and Dr. Watson when they visit the country house. The reader does not know whether to expect one or both animals to appear suddenly and attack the detective and his companion with loud screams and shrieks. Both Holmes and Watson show their courage by their intrusion in spite of the danger, not only from the animals, but from the formidable Dr. Roylott.
Helen Stoner tells Sherlock Holmes and Dr. Watson that her step father Dr. Roylott of the family of Stoke Moran liked to keep the following animals:
"He has a passion also for Indian animals, which are sent over to him by a correspondent, and he has at this moment a cheetah and a baboon, which wander freely over his grounds and are feared by the villagers almost as much as their master."
A cheetah is an atypical member of the cat family in that it cannot climb trees like other members of its species. It is the fastest land animal with a phenomenal speed of acceleration from 0 to 103 KMPH in just 3 seconds.
A baboon is a wild monkey.