You use the sneak button, which for me I set to 'S'. no, I did not know it until I asked the question on here and had to figure it out myself since so few people use mods.
If you play vanilla minecraft and asked this, there aren't any horses as you may have notice. If you download Technic stuff, (Technic is the actual software name), you get a bunch of awesome mod packs which have amazing features, and you can do cool stuff like ride horses.
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To dismount a horse, just use the left shift button. This is the same for getting out of minecarts and boats.
right click it.
First you must tame it, and then you just have to right click. Right click again to mount and left shift to dismount.
how do you tame a horse in minecraft 1.7.4?
Minecraft is supposed to be genderless. A horse can be either a boy or a girl if you want it to.