I was celebrating beating the champion by riding my Skiddo on the ranch. I walked into the long grass nearby, and a Moltres popped up!
It did the usual legendary Pokemon entrance, like Mewtwo's. I was so surprised that I just stared at it. After about ten seconds, it ran off, and I hadn't even pressed fight! I couldn't find it after that.
Why did it appear, and is it possible to catch?