I told the dolphin to "destroy the city" and immediately afterward it spit out the fireball. Also, a couple of attempts previous to my last, I got one to spit out a laser at a building on command to the same effect.

The attempt before the one last night was actually the closest. I had a whole swarm of dolphins flying in a coastal city after I told them to destroy it. I didn't see any destruction though. I don't know if they were rebelling or just flying around. All I know is there was a time skipping scene change and my dolphin got really tired. I got off it and summoned another one to ride. It was aggressive and bit my arm when I tried to get on it's back to ride it.