There are two rules addressing using armor for damage reduction in the CnC CKG. In both, the standard armor class rating is dropped to 10 (plus dexterity bonus and other miscellany) for all creatures. Armor no longer adds to one’s armor class.
So what does one do about monsters?
An orc has a standard AC of 11. Many humanoids have standard ACs and wear armor. The standard orc is assumed to wear leather or carry a shield. Hence and 11. The same is the case for many humanoids. So, say with an orc, goblin or hobgoblin, assume they are wearing an armor that equates to a +1 and use the rules as they are in the CKG (you may have to decide exactly which armor they are wearing for accuracies sake and to determine appropriate damage absorbtion).
But what about something like an imp. It has an AC of 15. One can assume a portion of this is related to size and speed. For small creatures I would throw out a +1 to AC. For fast creatures I would throw out a +1-3 for dexterity (this is really a gut call). For the imp I would give him a +2. The imp then would have an AC of 13. Its armor type would be padded or leather as it is an extraplanar creature.
The Jackal, which has a 12 AC and is a natural animal would move to an AC of 10 with an armor of leather. Many natural animal’s AC would be between 10-11 maybe on occasion more depending on the type of skin or exoskeleton they have (giant turtle being one that would be higher) or are very fast (hawk) or are small (viper). The armor type equivalent will depend upon the creature as well. A giant turtle would have better damage absorption (say breastplate) than wolf (about padded). Some animals have thick skin and a thick layer of fat (walrus or mammoth) making their AC low but DA about studded leather or laminar.
Take a night hag though. It has an AC of 22. It is an extraplanar creature but normal sized with no armor. This is a difficult one because that high AC normally makes them a challenge. When the AC ranges this high, an equitable AC and damage absorption must be created to meat the same. I would give it a +2 to AC for normally being 20+ ac extraplanar. I would also throw in a +3 AC for dexterity for an AC of 15. To determine its absorption is not a science (though I think I might be able to come up with a formula). But I would say it would be moderate, somewhere around the chain hauberk.