The phrase "wolf in sheep's clothing" describes someone who appears to be harmless or kind, but actually has ulterior motives or is deceitful. There are many synonyms for this phrase that convey a similar idea, such as "trojan horse," which refers to something that appears to be helpful but actually hides a harmful intent. "Snake in the grass" is also a common phrase used to describe someone who is deceptive and cannot be trusted. Another synonym is "undercover agent," which refers to someone who is pretending to be one thing, but is secretly working for another purpose. Regardless of the synonym used, the message is clear - beware of those who may be hiding their true intentions.