I havent really thought about special RR mods for creatures much before now, but at least concerning dragons their description should give alot of hints on what should have an effect, both to a higher and lowe degree.
For magic one could argue that since Dragons are creatures spawned from a good mix of Magic and the Elements they are somewhat effected to a degree. Already They are highly imun by one element (their own) and highly acceptive to another (the opposite). One could been say that elements and spell related to the Dragons core element receive mods from +/- 25 to say 50 depending on how much the spell is related or opposite of the core element of the Dragon.
Modifiers for other spells is again depending on how a GM feel the Dragons nature affect the the resistance to spells in his or her campaign world. In general I think most usually described Dragons as being highly resistant to magic which would justify a mod from 20 and up.
Dragons again being Magical creatures can as far as I remember eat and digest anything of this world and more I would imaging. This leads me to conclude that Dragons are not effected by mundane diseases and poisons, nor one which are forcefully pushed upon them by Magic, which would most like be Channeling.
As for other creatures it is generally up to the GM if there should be a mod or not. I wouldnt personally give additional mods to animals and other natural things unless they are inside or under the influence of an outside force, say nod or curse.
Beings with a civilization or basis for a tradition of magic could have their mods increased or decreased based on how much their pampering with magic of any kind have affected their race as a whole. For instance a highly superstitious culture/race might receive a penalty because of this factor and so on.
Elves for instance are usually described as one with the lands and highly magical which has been interpreted in a way such as their defences are slightly lower.
Basically what I am saying it is up to the GM in the end, also whether or not to make this considerations and calls ...