As he wanted to leave camel due to it's laziness, he wanted to let these three animals to do work for double time inorder to cover the work left by camel too.
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Actually the camel was lazy and he didn't want to work at all and because of this reason the man wanted the dog, horse and the ox to work for more amount of time because the camel wasn't working for the man. This reason has made all the three dog, horse and the ox angry.