I would also like to add the exercises of the classical gymnasium: shoulder-in, shoulder-out, travers, renvers. They all are a combination of stretching on side of the horse, flexing the other and strengthening the hindquarters at the same time. They can be varied and combined to make it actually fun and not just work. Going on a straight line or on a circle, you can smoothly change difficulty and stretching effect.
All these exercises have the added bonus that they can be done in increasing speed variations, whereas the usual stretching exercise are done stationary.
I think that those exercises, combined with those that Romy mentioned make for an ideal warm-up. And if done from the ground, they also warm up the human as well