"An Angus" is correct because Angus has a vowel in front. Thus it is proper grammar to insert "an" in front, not "a." If there was a consonant in front, like the "b" in Brangus or the "s" in...
An Angus is: #1: An ancient breed of Highland Cattle colored either Black or Red. #2: A district in Scotland, largest town Arbroath or Aberbrothock. #3: A film made in 1995.