I've had giants slay dragons for me. I've also had guards pitch in, and sometimes I'll have one of the companions with me,
My very first playthrough, I had gotten to the point where I could have some of the Companions for followers, and that faction was pretty much the only thing I'd done besides bits of the MQ. So I asked Vilkas to follow me just to see what it was like (because I'd had Lydia up to that point and hated her). On the road between Whiterun and the Western Watchtower a dragon attacked. All the guards in the area come rushing in, I'm doing what I can, I think it may have been my first dragon since Dragon Rising so I was really flustered and just trying to hang in there as long as possible.
The dragon finally landed and before I even had time to react and get in near it (I was doing mostly melee at the time), it gave a big death flop and that was all she wrote. I look around for Vilkas and don't see him, altho I knew he'd gone into combat mode when the thing first attacked. I stand there and absorb the soul, then walk towards it to collect my loot, wondering if I'm going to find a dead Vilkas somewhere in the vicinity. Suddenly a guard who is standing there turns to me and says,
Spoiler
"Maybe I'm imagining things, but I could swear I just saw a werewolf."
To this day I still don't know if Vilkas turned into a werewolf and went nuts on the dragon in beast form and that's why it died so quick. About the time I realized what the guard had said and the possibility clicked in my head, I turned around and Vilkas ran up from wherever he'd been or gone.
Has anyone ever used Aela, Farkas, or Vilkas as a follower and had them use the Beast Form to fight something? I traveled with Vilkas a lot on that character, even married him, but I never saw him transform with my own eyes. I don't even know if such a thing is possible, if those NPCs have a choice about it or only turn when they're scripted to (like Farkas in Dustman's Cairn).