I was riding up to Bravil one day, and a Will-O-Wisp appeared. Of course, I didn't want to fight it, so I kept riding. It chased me all the way up to Bravil, where the real problem started. I had picked up two wolves and a mountain lion on the way, and it formed a big Benny Hill train of death. Nearby NPCs wanted to fight, and the Wisp retaliated. All in all, it left my horse, the guy who works at the stables, the wolves, the mountain lion, and two guards dead. I gave them all a ceremonious shove off the bridge.
The Benny Hill reference made me LOL
I've had a very similar thing happen on the way to Bravil from Leyawin. Think it was a Troll, two mountain lions, and a boar, then two guards in my case.
I wonder . . . if you just kept riding around Cyrodil, pausing occasionally to let stragglers catch up, how many monsters could you accumulate into a "convoy." Put Yakkety Sax to it as a theme song and you'd have one heckuva good video.