The bible says the tail moves like a cedar, and I would think they were referring to movement of the branches.
Incidentally, the hippo has an interesting use for his tail. I was at the zoo once and saw a hippo poop with his butt about 8 feet from a wall. As he pooped, he spun his tail leaving a 10 ft diameter spatter pattern on the wall. My immediate thought was the old saying: where were you little man when the s... hit the fan? On further thought I realized that this would be a good method of dispersing the poop in the water to be carried away, rather than leaving it in large globs to contaminate the pool the hippo lived in.
I now remember seeing something like you describe! Totally hillarious!