I've been going back through my older books and then forward into RMSS, because I thought there was something that explicitly said creatures don't parry, but their DBs reflect their natural tendency to dodge. I can't find it, though.
I have never allowed non-humanoids to parry, and I don't think I would. The premise of parrying in RM2 is that an individual has a weapon that they have learned to use in combat, and that the weapon has some level of tactical value for blocking strikes from an enemy's attack. If you look at the rules for parrying, it's pretty clear that parrying is more than just re-adjusting one's focus from attack to defense. One's skill with their weapon is crucial, as it differentiates a parry from just using a weapon as a shield.
Presumably the original rule-makers were very aware that they had put all of the rules in place for parrying, and could have made similar rules for creatures that don't use weapons. But they didn't. Instead, the creatures get a DB that reflects their natural speed, and modifiers for flee/evade and charge/lunge (although how those modifiers are intended to be used is NEVER explained! Grr.) The lack of any reference at all to parrying for unarmed creatures is a contrast that says a lot.
In my mind, I see three (four?) specific types of defense:
- Natural defense based on ability to dodge and move out of the way (QU-based DB, for both humanoids and creatures)
Practiced evasion (Adrenal Defense, comes with its own limitations and restrictions)
Unpracticed blocking (weapon or other item as shield, flat item-bonus)
Practiced defense with a weapon (parrying)
As one last point to support my case, consider the shrew. I use them in demo combat for new players, to point out how deadly RM can be. A shrew is AT(DB) 11(70). A mole, similar in size, is just 1(30), as is a rat. The description refers to a shrew's "extremely tough skeleton," which is presumably the basis for AT 11 instead of AT 1. But that DB has to represent its natural speed, "hyperactive existence," and evasiveness, and how readily it evades being struck. Allow it to parry with (all or part of) its 45 OB and that sucker's going to kill your party.