Neither.
They developed independently for many years, they have a different focus (pets and lifestock), and look identical because the same software is used now - (there aren't as many options as before social media got big), and I think by now they have same owners. It's distincly different crowds, and both are very good rabbit related forums.
Not a copy, not a spinoff.
I rather like Rabbitsonline, only pet forum I don't need to hide that I breed for meat too, but I respect that most there don't like the idea and do not rub it under everyones nose, but no need to lie about it (got kicked from other boards because I'm breeding rabbits)