I don't think it's explicitly stated in RMSS. The only place the -20 penalty is listed (that I could find) is in the description of Two-Weapon Fighting. So if you think creatures with multiple attacks are just a special case of Two-Weapon Fighting skill, the -20 would apply. If it's some kind of inherent ability, it might not. Personally, I think due to the size differential, it's going to be easier for a dragon to attack multiple targets than it will be to hit the same target with multiple limbs, head, wings, and tail. That might not be the case if a dragon is fighting against, say, a pair of titans.
In RMU, we do treat all multiple attacks including creatures as a special case of Multiple Attacks skill, so the creature would have a bonus in that skill and the normal rules for multiple attacks would apply.