I know it's not an airline callsign however one of our Air Force Squadrons used "Wombat" followed by the task number as a callsign.
The Wombat is a vegetarian nocturnal marsupial that eats roots and leaves. Apparently the squadron members identified closely with this.
Ansett Air Freight also had the wombat painted on the nose of their Electra freighters in the 70's and early 80's.
CC