black sheep

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A sheep with black fleece.
• noun One who is considered disreputable or disgraceful by his or her relatives or associates.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Used other than as an idiom: see black,‎ sheep.
• noun idiomatic a nonconformist; an unusual or unconventional person
• noun idiomatic a disliked person; one who is disfavored

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a reckless and unprincipled reprobate
• noun sheep with a black coat