Riding cattle is generally seen as a novelty or tourist activity. It is something many kids (or older folk) on a farm might try to do at some point. Just because, well, the cows are there.
In general cattle are much better as draft animals, pulling a cart. Unlike horses they are reluctant to move quickly and they have very wide bodies. However with patient training cattle will learn to jump fences and carry out other more athletic activities.
So most people use cattle for riding largely due to the lack of a better candidate. Horses have a much smoother movement which is far less jarring to the rider.
But as a challenge or parade novelty a steer or buffalo does make an impression.
One famous example (see below) dates from 1939 and the New York World's Fair Texas Jack, and his mount a bull called Scandalous John travelled from Texas to New York to appear at the fair.
However, steer riding actually has quit a long history and steers can make very practical mounts. Native American Blue Duck is reputed to have ridden cattle on raids as their tracks were more easier concealed.
There has been a revival in this activity with some cattle such as longhorns being raised specifically for saddle use.
But if you look at the barrel "racing" steer below, you might see why a horse is generally a better choice.
And of course there is the kind of cattle riding where the difficulty of the task is the whole point: rodeo bull riding.
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