Water Polo

Chess without horses is just lacking something

Diving. I'm still not convinced that this is real: https://en.wikipedia.org/wiki/Diving_horse

Texas Hold 'Em

Archery

hobby horsing

Ski jump, obv.

Basketball

It kind of already exists
https://en.wikipedia.org/wiki/Buzkashi

Biathlon. Would love to see a horse fire a rifle.

Skydiving.

Nascar

This thread is terrible, the obvious answer is surfing.

Greco-Roman wrestling.

Curling, ice skating, synchronized swimming. It'd be hard to find a sport that wouldn't be improved by horses.

Ice Dancing

Polo is Croquet on Horseback

Pole dancing?

Water polo seems cruel to the horses.

Chess.

I wanna see two nerds trying to castle while riding at a full gallop, trying to juggle the board on the back of a saddle like a first-time dad trying to tell his wife to hold on until they get to the hospital.

Extreme ironing - and yes it is a sport. WE WILL NOT BE MARGINALISED!

Tennis

Shooting. Preferably with an equestrian event style course. Would be rather entertaining I think but spectator seating would be…. Complicated.

Snooker.

Rugby

Baseball

Bowling

Synchronized swimming.

I vote paintball with horse armor.

Also the markers are mounted on the horse, turning them into agile paint tanks.

Taekwondo

Ice hockey. I mean, ice polo. The fights would be wicked.

Hobby horsing

I do not enjoy hockey, but let's not for a second pretend that Polo is better than hockey.

Donkey basketball is enough of a thing that I’ve seen it played multiple times with my own eyes.

Although it’s mostly for comedy rather than genuine sport.

Ice hockey

Bobsledding.

"Feel the horses, feel the rhyme, get on up, it's bobsled time"

Is your question implying that you think polo is better than hockey? No way!

Math Olympiads (if the horses had to answer the questions, like Clever Hans).

Fencing or pole jumping

Baseball

Archery