You hit them up the rear end then it's your fault regardless of what they stopped for, even if they think it's their fault.
To correct ugly bob, there is no such thing as "right of way", it is not even mentioned in the highway code, instead there are certain situations when a road user should "Give way". Therefore an animal cannot be credited with having "right of way" or even the awareness to "Give way".
There are rules when you should brake for an animal, I'm not sure of the exect specification but for example you should brake for: Dogs, Deer, Horses, cows, sheep etc you should not brake for: Cats, squirrels, other rodents, birds. Not sure about foxes. However braking or not does not remove the onus on following vehicles to leave a safe distance.
Now technically you don't have to brake for Rhino but I recommend that you probably should!