First off, I saw the background posted in the other thread, and I think it's really cool
(Also like what you did with the art for Dire Rhino)
However, I do feel that it doesn't deserve a cost increase (just my intuition)
There is one main problem, though, and that's with the way it's worded. It appears to be a passive ability, but not one that's limited to a one-time use. So, every turn each rhino attacks, it's getting weaker. If you could, (or intend to) add to the notes that it resets.