We used to have two horses turned out together in a large field. One was essentially retired - so he was only brought out a few times a week. The other was getting ridden 5-6 times a week. So I'd go out to get the one to ride, and the other would walk away all the time and no one thought twice about it until... someone went to grab the older horse for the farrier and he had learned that by walking away he didn't have to get caught. So of course they walked after him and he took off to the other end of the field (roughly 600ft)... And then the other horse started running.
So then they both think this a great game! To catch either of them for a few weeks after that we had to corral them both into a small pen at the front of the field. This required 3 people, one to go down to get them running (aka walk towards them), one to get them into the pen and another to close the gate. Once we got them in the pen, they stopped and just stood there -_-
Eventually we broke the habit - thank goodness!