Jessabel, my collie cross play with my Standardbred in the paddock! :) It's so much fun to watch. When the collie arrived, he got too close behind the horse while I was riding and found out about hooves, so he was scared for a while. Then one day, the horse was playing and the dog thought, "Gee, that looks like fun!" So he went to play too. Well, the horse didn't know what to think of that! So, she tried to chase the dog away. Didn't work of course. The dog stopped when I called him.
Fast forward a month or so, and the horse and dog were playing again. Then my son called the dog to go for a walk. The horse went to the fence and looked out, "Hey, where are you going? Aren't we playing anymore?"
Now, if one of them stops playing before the other, the playful one chases or barks the "lazy" one until (s)he plays again! Too funny! I love watching them.