Theory:

     Then Grandfather, just to get his own back, took from his pocket our pet tortoise, and said, “What must I pay for this, since you charge for all animals?”
 
     The ticket-collector looked closely at the tortoise, prodded it with his forefinger, gave Grandfather a pleased and triumphant look, and said, “No charge. It is not a dog.”
 
     When Toto was finally accepted by Grandmother he was given a comfortable home in the stable, where he had for a companion the family donkey, Nana. On Toto’s first night in the stable, Grandfather paid him a visit to see if he was comfortable. To his surprise he found Nana, without apparent cause, pulling at her halter and trying to keep her head as far as possible from a bundle of hay.
Explanation:
 
Grandfather was annoyed by the ticket collector's behaviour and was unhappy with the three rupees he had paid for the monkey. He took out his pet tortoise from his pocket, angry, and asked the ticket collector if he charges for that as well. The ticket collector paid close attention to the tortoise and pressed his forefinger against it to examine the creature's type.

After inspecting the tortoise, he smiled at grandfather and said that no ticket was issued because it was not a dog. Nonetheless, grandfather had been concerned about paying three rupees for the monkey's ticket.
 
When the narrator's grandfather returned home from Saharanpur, it seemed that his grandmother had known about Toto. Nothing as bad had happened as Grandfather had predicted. Toto was given a position in a stable after Grandmother accepted his presence in their house. Grandfather kept it with their family donkey, Nana, because Toto wouldn't let other animals sleep through the night.
 
Nano.jpg
Nana
 
Since grandfather was aware of Toto's naughty behaviour, he visited Toto's place to see if it was comfortable. He also wanted to see if Toto was okay with being around their family donkey. He then noticed that their family donkey, Nana, was skipping meals by standing away from the hay bale for no apparent reason.
 
Meaning of difficult words:
 
S.No
Words
Meaning
1.
Prod
To push or press something with your hand
2.
Triumphant
The feeling of being joyful having achieved a victory
3.
Stable
A building where horses or  cattle are kept
4.
Apparent Able to be seen or understood
5.
Halter A piece of leather strap or rope that is tied around an animal's head
6.
Hay Grass that is dried and used as an animal food
Reference:
National Council of Educational Research and Training (2006). Moments. The Adventures of Toto – Ruskin Bond (pp. 7-10). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.