Klingons because their forehead ridges look like a fanny.
From Family Guy:
Captain Picard: No.1, if I were to whisper in your ear that Commander Worf's head looks like a fanny, would you join me in a laugh?
Riker: Yeah, I could get into that.
CP: All right, here it comes... (yells) Commander Worf's head looks like a fanny!
Worf: You can both suck my ridges!
CP: Oh, get a sense of humour, Rocky Dennis!
http://en.wikipedia.org/wiki/Roy_L._DennisFamily Guy or Simpsons