Green’s Dictionary of Slang

(Aus.) a dog, not necessarily of mixed breed.

1922		Smith’s Wkly (Sydney) 4 Mar. 23/2: He done his block, yelled like a tom-cat, and ducked to the shelter-sheds like a mong with a tin-can on his tail.
1930		Western Mail (Perth, Aus.) 13 Feb. 39/1: He always had time [...] to have a frolic with ‘Mong,’ as he had named his collie after a school companion had called it a mongrel.
1941		Hackston ‘The Horse from Bungowannawinnie’ in Mann Coast to Coast 163: No mong’s going to heel that horse.
1944		A. Marshall These Are My People (1957) 52: He had the blood of all the mongs in the district running through his veins.
1968		J. O’Grady Gone Troppo (1969) 16: Poms and Pekes, Poodles and Pugs [...] Mutts and Mongs.
1978		T. Davies More Aus. Nicknames 69: Mong (Short for Mongrel Dog).
1988		McGill Dict. of Kiwi Sl. 73/2: mong mongrel.
2003		McGill Reed Dict. of N.Z. Sl. [as cit. 1988].