Punchy Pooch (1962)

At the carnival, Champ is persuaded to enter a boxing attraction with "The Australian Bounder", a fighting kangaroo. The winner receives $500.00 if he can stay 4 rounds with the animal. Doc and Champ are pleased to accept but the kangaroo and its manager don't exactly fight fair. To help Champ win, Doc comes up with a variety of schemes such as convincing the kangaroo that Champ is a mother with child, then that Champ has the measles, and, in the final round, by putting Champ on a unicycle so he can outrun his foe.

Unfortunately the movie Punchy Pooch is not yet available on HBO Max.