Theory:

     Presently the Dog came to him, with a stick in his mouth, and said, “Camel, O Camel, come and fetch and carry like the rest of us.”

 

     “Humph!” said the Camel, and the Dog went away and told the Man.

 

     Presently the Ox came to him, with the yoke on his neck, and said, “Camel, O Camel, come and plough like the rest of us.”

 

     “Humph!” said the Camel, and the Ox went away and told the Man.

Explanation:
  
Next came along a dog, with a stick in its mouth. He also asked the camel to go along with him, to carry and bring goods like the other animals.
 
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The dog also returned to complain to his master.
 
As we already know, the camel only replied "Humph". The dog also went to the man to complain about the camel.
 
Then an ox came to the camel, to try to make the camel do some work. The ox had a wooden instrument fastened over its neck. This will be usually attached to the plough or the cart that ox pulls when they plough the fields.
 
yoke-575205_1920.jpg
Yoke, fastened on ox's neck, to facilitate ploughing.
 
He asked the camel to go and plough like the other animals. The camel again replied "Humph". The ox returned to the man to complain.
 
ox.png
The ox also returned to the man.
 
Meanings of difficult words:
  
Words
Meanings
fetchgo for and bring back
ploughturn up the soil, especially for farming
yokea wooden crosspiece that is fastened over the necks of two animals and attached to the plough or cart that they are to pull.
Reference:
National Council of Educational Research and Training (2008). How the camel got his hump - Rudyard Kipling (abridged) (pp. 01-06). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.