“披着羊皮的狼”用英文怎样说？

"A wolf in sheep's clothing" is the better translation, because the word "clothing" EMPHASIZES what the Wolf is WEARING on top of its own natural skin. Please note there is no "a" in front of the word "sheep" which is in plural form (复数名词) in this phrase.

Example:
"I want you to stay away from that boy. He's a wolf in sheep's clothing."

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The Wolf in Sheep's Clothing
from Aesop Fables《伊索寓言》

A Wolf found great difficulty in getting at the sheep owing to the vigilance of the shepherd and his dogs. But one day it found the skin of a sheep that had been flayed and thrown aside, so it put it on over its own pelt and strolled down among the sheep. The Lamb that belonged to the sheep, whose skin the Wolf was wearing, began to follow the Wolf in the Sheep's clothing; so, leading the Lamb a little apart, he soon made a meal off her, and for some time he succeeded in deceiving the sheep, and enjoying hearty meals.

Appearances are deceptive.

A Wolf who covered with the Sheepskin

A Wolf covered with Sheepskin
It's Right!!!

Be covered with the wolf of the sheepskin