Beautiful ugly is a term used in modeling, when a model has such striking or odd features that they could be considered quite beautiful or very ugly. This is especially rare, and highly desired, especially in couture shoots.
Very few "beautiful uglies" exist, as many are simply labled unatractive, and unsutable for any type of modeling.
A very good example of such a person is Omahyra Mota, a Dominican supermodel. Her beauty (or lack there of) is completely in the eye of the beholder.
Omahyra Mota looked quite ugly in X-Men 3, and she sort of looked like a transexual with that odd hairdo.
Although, she looked quite pretty and punk in Omahyra and Boyd, so I guess she's a beautiful ugly model.