If the autistic child is very verbal, then try to talk about what they want to talk about. But, my experience with autistic people is that they dislike someone talking AT them, especially if they would like silence to concentrate on the horse. So, is it necessary that you talk?
if so, for the autistic kids you can direct their attention to something rhythmic, such as the steps. if they are older, can you get them to count out every time the horse steps with his front right foot?, or to feel the barrel swing side to side.
or, for non autistic kids or those that are like to talk while riding, you could ask them to guess what the horse had to eat that day, or , "I wonder what horses dream about? ...." how old do you think Brownie is? is that old? Do you think Brownie would look good with a haircut?
I dunno, just silly, imaginative things. It really depends on how old these kids are.