6-18 is an anthropomorphic dog of unknown gender.
A cousin of 47-521, 6-18 was one of the hundreds of Beagle Boys who swarmed Duckburg when they heard tell that their local relatives had finally taken control of Scrooge McDuck's fortune, wanting to get a piece of the pie.
Behind the scenes
6-18 is mentioned in the 1975 comic story Scrooge For A Day. They are presumably one of the background Beagles seen later in the story, but which one is unknown.