I was running around in the Nether trying to hunt ghasts for ghast tears. When I saw a ghast in the distance, since I was using my elytra I would keep going for a second to land then turn around and shoot it with my bow, but when I go back to where it was just a second ago, it is gone. This happens to me all the time with every mob. I saw a herd of sheep in the distance and kept running then remembered I needed wool, so when I ran back to them they were gone.