Gnasher

A black dog was a character in The Beano.

He appeared on the front and back of The Beano Summer Special 1981 with Dennis the Menace at Dolphinarium, a swimming pool. In these depictions, the front depicted him with Dennis on a dolphin wearing a cowboy hat labelled "JR's", and using a water pistol against a group of individuals. The back depicted them both in the air after another character held a hoop on a stick for the dolphin to jump through. (TV: The Rings of Akhaten)

Information from invalid sources
Another source concurred regarding the depiction of the character on the cover of The Beano Summer Special 1981. (NOTVALID: The Universe is Calling)

Other matters
Although he was not named as "Gnasher" in TV: The Rings of Akhaten, he is named as such in the real world, and was the pet dog of Dennis the Menace.