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There is a famous saying in Tamil language: "Paarthaal pasu pol, paaindhaal puli pol" which translates to "Looks like a cow, leaps like a tiger".
This is usually used to describe a person who on first look gives an impression that they are silent and submissive, but when confronted, they dominate the confronter.
The closest English proverb which I could think of is Appearances are deceptive, but it doesn't essentially capture the silent nature and the combative nature of a person.
Is there an English saying which means the same?