Nag; was a big, black venomous snake; specifically a cobra
Rikki-tikki was a mongoose, a type of animal that feeds on the flesh of other animals especially snakes
Darzee and wife who were tailor-birds were greatly afraid of Nag, who was a cobra, a venomous snake. Nag believed that he was the king of the jungle. He went about inflicting fear in other animals that lived in the jungle where he was located.
Furthermore, one of Darzee's babies had mistakenly fallen from the nest the previous day and Nag had eaten him up. This made them very sad and miserable at the death of their baby. And they feared Nag the more.
Hence Darzee and his wife greatly cowered from Nag because they were afraid of what he could do to them. That is Nag could kill and eat them up.
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