1) I see no reason to specifically exclude horses or camels. It'd give a nice "oh, I can identify that" thing. I think we should keep 'em unless you have a need to remove them. I'd like to keep horses definitely, because the nomad plains will need something. But I suppose I could come up with another beast of burden if it comes down to that.
2) griffen. predator, obviously. likes to eat horses and camels, incidentally, or other cattle (unless there are no camels, then well, it has to eat some other largish animal). The role it fills is essentially that of a flying lion or other big cat. So it's a lot more efficient. That means, for ecological balance reasons, it needs to be much more rare than a lion would be. Perhaps not all their eggs mature and hatch, so they don't multiply so quickly. So, I'd sum up with common enough that people know they're not myths, at least, people near their environmnets. But definitely not so common that you see one every day, unless you live real near a nest (in which case, don't try raising cattle, and in lean years, make sure they don't come for you).
This pretty much applies to any griffon. If they have the fore parts of a bird of prey, then their method of getting food is essentially carnivorous (sharp beak, talons), so even if they have the hindquarters of an herbivore (like a hippogriff), they still prey on animals, and big ones if they're horse/lion/whatever sized. (I suppose you could have a squrrill-griff which preys on... rabbits? Maybe cats and dogs if it feels ambitious -- which gives me an idea....)