Visitor (South Park)

Visitors (often called Greys) are aliens who came to Earth and gave a Human named Eric Cartman an anal probe. They also later kidnap Kyle Broflovski's adoptive brother Ike Broflovski. As a matter of fact, Visitors believe that cows are the most intelligent species on Earth, and considering the fact that they were studying South Park, it would seem they are rather justified in thinking this, if not totally accurate. They only say "Moo" after having a talk with the cows when they think it's some kind of new language.