OK K.O.! - Short Episode 6: Enid's Bad Day

https://www.youtube.com/watch?v=_QLCiwQ6Mgg

A crowd of impatient customers get on Enid's bad side. One background girl is seen wearing boxing gloves,

Snoopy, Come Home (1972)

Snoopy accepts Lucy's challenge to a boxing match. Pretty one sided with Snoopy scoring most of the 'punches'.

The New Batman Adventures - Season 1 Episode 1: Holiday Knights

Poison Ivy punches Batman with an oversized boxing glove. One punch. Quick scene.

Princess Punchalot

https://youtube.com/watch?v=eXr7TZybhJc

Princess Punchalot fights Throwafista to return her kingdom's stolen core gems. Good back and forth. 

Web Animations?

Trying to expand the criteria to amateur (but still decent looking) animations. Stuff from YouTube, Vimeo, dailymotion, etc.

Please provide original sources from the creators themselves.

SpongeBob SquarePants - Season 1 Episode 11a: MuscleBob BuffPants

Sandy Cheeks agrees to help Spongebob get buff. A quick boxing match that Sandy ends with one punch.

Popeye - Never Kick a Woman (1936)

Popeye takes Olive Oyl to a sporting goods store to learn self defense. The female boxing instructor flirts with Popeye, and the women engage in an impromptu match. Evenly fought bout between the two.

Total Drama World Tour - Season 3 Episode 6: Aftermath: Bridgette Over Troubled Water

Geoff and Bridgette are put into a boxing match to resolve their feud, but end up rekindling their old flame. No real fight.

Total Drama World Tour - Season 3 Episode 18: Aftermath Aftermayhem

Leshawna must box a kangaroo in a challenge. The fight lasts one punch.

The Fairly OddParents - Season 2 Episode 3: Hex Games

Vicky shows off her skateboarding skills by multitasking. Quick speedbag segment.