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I'm writing a character for a fantasy setting, and I wanted him to be a gruff half-orc war master whose mount is a bull. I wanted to know if there has ever been an actual historical use of bulls or steers used as mounts, and what are some of the challenges that come from it so it could be covered a little in my story.

As it's for a fantasy setting, there can easily be some suspension of belief. I wanted to add some flavor to the story and to the character by trying to address some of these challenges.

I'm delighted (and astonished) to say yes, not only has it been done but it is being done as a business.

Kimberly and Annette here demonstrate just how well behaved their broken steers are.

YouTube video of Kimberly riding Jazzy, and Annette driving Dozer.

They make appearances at birthday parties, visit senior centers, go to
schools and community centers and of course are ridden and driven in
parades.

An article in Horse Nation (of all things) tells how she finds the experience:

“They’re a lot more ‘whoa’ than ‘go,'” she states. Friends who have
ridden gaited horses tell her that the steers feel very similar,
smooth but ground-covering. They fit into saddles with extra-wide
trees; Kimberly rides in both western and dressage saddles with
draft-sized girths. One major difference in tack is the nose bit:
rather than a bit in the mouth like a horse bridle, saddle steers are
guided by a metal bar that’s pierced through their nostrils (similar
to a nose ring for driving oxen.)

I don't see much practical use in riding a bull instead of a horse, they are slower, for one, with an average top speed of just around 40km/h compared to a horse which has a top speed of 88km/h, and are considerably dumber than horses.

Not to mention the most crippling problem, their eyesight.

• Field of Vision-
  When cattle have their heads down to graze, they can see almost 360 degrees. As they raise their heads, the sides of their bodies create a blind spot behind them. This panoramic vision allows them to see in all directions without moving their heads, but they have binocular vision only for an angle of about 25 to 50 degrees in front of them. Binocular vision is like ours: Both eyes focus on an object, allowing the perception of depth, speed, and distance. Their monocular vision to the side allows them to see movement, but it is not sharply focused like human vision.


• Depth Perception-Cows have slit-shaped pupils and weak eye muscles, which means they cannot focus quickly. They have poor depth perception because they have limited vertical vision of about 60 degrees, compared with around 140 degrees for humans. Because of this, they do not like to walk across shadows as they cannot tell if a shadow is a hole or ditch; they will stop moving forward and lower their heads to the ground to check it out.

If your character is heading into battles or even war, the bull would not be as reliable as a war horse which has been bred for centuries for the purpose of carrying soldiers.

Now regarding the orc. I'm going to make assumptions here and say that the half-orc would be around half of what an orc would weigh. According to the wow wiki (which I used as my reference) Orcs weigh between 226 - 652 lb. I'm going to assume the worst and go with the upper bound of 652 lb, which divided by two equals 326 lb.

I am also going to assume that the Orc is wearing some sort of armour, and I'm going to use common medieval iron plate armour as my reference. With some searching, the average weight for full plate armour I found was between 33-55lbs, and since I'm assuming that the half-orc is bigger than a human, maybe upper 6 to 7+ feet tall, I'm going to add a few extra pounds to the upper bound, which comes to around 60 lb.

(I am not going to take into consideration any extra armour/weapons/food the half-orc is carrying with him, for simplicity's sake.)

Summing it up, the total weight this bull would have to carry would be roughly 386 lb on its back. This may seem a lot, but apparently, not quite.
According to this page, the Barns-ford Ferny, a Charolais bull, weighing at around 4,000 lbs could easily pull 4,000 pounds at a walking pace.

Taking this fact into consideration, it seems very possible for a bull to carry the orc along with iron plate armour, easily, whereas horses are only really supposed to carry no more than 20% of their own weight. (The heaviest horse in history was the shire horse, which weighed 3,360 lb, 20% of which still isn't enough my estimations for your half-orc)

And regarding the history question- No.

Summary: It is possible for the bull to at least carry your half-orc into battles or just to a town etc. equipped with all his miscellaneous wares. It is, however, not recommended to use a bull for battle, as they are slower, dumber, and generally more unreliable to horses.

Water buffalo! Yeah!

http://www.ezytourthailand.com/packages/Buffalo-Racing.html

If you want to see some amazing images, google up the annual Water Buffalo race in Thailand. Some are being ridden solo like this one and there must also be a category for 2-buffalo chariots. These water buffalo are cooking along. This is not a rodeo - by the number of participants these buffalo must be trained to race.

Water buffalo are bigger and stronger than cattle and the bulls are more docile and tractable. They have been domesticated for as long as cattle. Google image suggests people ride them not uncommonly, including a very pretty Thai girl on a buffalo with such big horns I wonder if they are fake. Water buffalo can run 30 miles an hour. There must be some reason why they were not historically used for cavalry but I can't figure it out. Possibly fear of too much awesomeness.

Your halforc should definitely ride a water buffalo. Or better - dude is not littler than the average orc because of his human blood but bigger, like a liger is bigger than a lion or a tiger. He is a burly beloinclothed bear-sized HO; larger than the largest orc and so he must ride a chariot pulled by 2 water buffalo.

Don't worry about it.

Your character is an orc in the first place. It is unnecessary for the bull to adhere to rules of a rodeo bull or any of the more docile bovine species bulls.

I mean, people accept wolf riders in fantasy stories, a bull should not be that hard to tame for him, since it doesn't even need to be a "western" bull. Depending on your orcs, it's likely to have to be far larger and fierce than a western bull anyways. If in your story setting it's uncommon for this to take place, then you can take even more liberties with it because the other characters will view it as weird and noteworthy.

Yes, but, well, good luck.
For one reason or another, bull-riding is a professional sport. The goal is to stay on a bull for as long as possible. 8 seconds is considered a "win." So theoretically, yes, but a word of warning: Bulls are hard to control.