I'm currently trying to build a world where there is no animal that is like a horse, for example mule, or donkey, or horse.

What animal could be ridden by humans if the animals were the same size as a horse? 

For example, what if there was a big spider that could be ridden, or a wolf that has an acceptable size to travel with a man.

On top of that, how would men battle with this kind of rideable animal, and tools that will be used: saddle, weapons ... ? Also, how well will humans be able to travel over long distances?

I think that Dinotopia is a good example to illustrate what I am looking for, but without dinosaurs.

**Edit:** I'm not sure I was totally understood. Here I'm not looking for existing animals that are horse-sized, but animals that normally are smaller. 

What kind of small animal have a good temperament, Strength and stamina to be ridden. Only mammals? or insect could be chosen? The horse-sized duck is a good one because it's amphibious.
**For example**: could a horse-sized squirrel be a good mount?