Animal acting the parts of boxers are a traditional part of circuses and sideshows. The animals do not really try to hit each other, but rather put on a act that looks a little like a boxing match either with another animal or a human trainer.
Kangaroos actually have quite small and weak front legs. But they will natural make a sparring motion when approached or challenged (see video below). With boxing gloves on the effect is quite realistic. And of the kangaroo really wants to hurt someone, they will use their strong hind legs.
"Boxing" or wrestling demonstration seem to have sprung up as soon as kangaroos were brought to Europe in the early 1800s. Since then many circuses had a boxing kangaroo as it is a flexible combination or exotic animals exhibition, animal training act and comedy.
There are a small number of acts still including boxing Kangaroos such as the Eastborne Circus and Piccadilly Circus (United States). This act has been vehemently protested by PeTA.
The boxing kangaroo has become an unofficial symbol of Australia. It is widely used to represent Australian supporters at sporting events.
This symbol is actually a trademark of the Australian Olympic Commission.
Its appeal rests not in a literal interpretation but because it combines a native animals with a competitive "fighting" stance.
Bears of various species have been exhibited boxing either with each other or against people.For example the show above is from the 2004 Spring Festival in China.
Circuses would create an entire tableau of boxing with chimpanzees including a chimpanzee referee and nurse. (e.g. Boswell Brothers, 1940s; Bertram Mills Circus, Polack Brothers Circus, 1950s).
A Thing of the Past
Animal boxing acts are now largely obsolete as people are more interest in seeing exotic animals acting like animals rather than imitating people.
Also there is discomfort with even the appearance of violence directed towards performing animals or anything that looks like animal fighting.