Not really as a species but to individual horses i would ask-
1)How comfortable is your tack?
2) On a percentage basis how much of the time do you actually understand what your owner is asking you?
3) Do you think that you have a give and take 2-way relationship with your owner or do you feel they take more than they give?
And i would also want to know- what do you really think of being stabled. Actually im going to stop there because there are loads of things i would ask and thats one more than asked for and this will turn into an essay.
1. Why do you put up with such terrible treatment from some humans when you are so much bigger and stronger than we are? I know the answer to this - horses are herd animals and it's preferable to be in a herd, safety in numbers, and be badly treated, than to risk being alone - but sometimes I am so shocked by the things I hear/see/read that it just makes me want to ask "why do you put up with it?"
2. What is the one thing you would really want us to provide you with that we currently don't?
3. This may sound really silly but I'd like to ask what they think happens to a horse which is sold, and therefore suddenly not there anymore. Do they think it has died? Or that it has left the herd? Obviously they'd have no concept of it being sold so it would be fascinating to see how horses register that horse suddenly not being around.