Earlier this year, I designed four monsters for the Giant Slayer adventure path. It is a great honor to be able to leave my mark in Pathfinder canon, and I hope that none of the four get cut out for space reasons. Naturally, I wanted to make as good monsters as I possibly could, so I put a lot of research into each of the monsters. I've created monsters before, but when you're designing something for your own gaming group, you don't have to think about balance or other aspects of monster design quite as seriously—what mostly matters in homebrew design to me is that everyone's having fun at the table.
Most of my research involved browsing the srd to see what commonalities similar monsters have and how similar monster abilities work. I did my best to imitate the Paizo style in every aspect of my design. One of the things I noticed was that there are many patterns that Paizo's monsters never or very rarely break even though it isn't explicitly mentioned anywhere. You just have to know.
This is a mini article so I won't even try to make an exhaustive list of these "unwritten rules". Besides, I'm still a fairly new freelancer and there are probably many unwritten rules I'm not even aware of just yet. So I'll just mention two examples to give you an idea of what I'm talking about. The point is, follow the patterns unless you have a very compelling reason not to do so. More often than not, it's a mistake to break the pattern.
Natural attack typesAccording to universal monster rules, the table "Natural Attacks by Size" lists the most common natural attack types. The last row is called "Other" suggesting that monsters may have custom natural melee attacks. However, if you pick any 100 monsters from the srd at random, you'll notice that it's exceedingly rare for Paizo monsters to have custom names for natural melee attacks. (Ranged attacks are different, though, there is no standard set of attack types.)
It's very tempting to give a tree-like creature a "branch" or "vine" attack, for example, but if you have a look at the treant or the assassin vine, you'll notice that both of them use slam attacks. It doesn't mean that all creatures with slam attacks have similar appendages; rather, the attack type only describes how they use their limbs to attack.
Why does Paizo use the standard types of natural attacks (almost?) exclusively? One of the reasons may be that there are a number of abilities and spells that interact with certain types of natural attacks. If you have a custom type, no such ability can affect it. More importantly, using a standard set makes the rules more consistent. You don't have to explain what damage type the attack does, for example. In other words, you save a few words.
Damage reductionNo Paizo monster that I know of has DR 25 or above. Barring truly extraordinary circumstances, DR 20 seems to be the cap.
A lot of dragons have DR 20/magic. Other types are rare, but exceptions exist such as the quantum golem with its DR 20/adamantine and bludgeoning. That's much harder to bypass, but then again it's a CR 20 creature so it is less of a concern. DR/magic, on the other hand, is the weakest form of DR since pretty much everyone has a magic weapon by the time you start fighting wyrms and great wyrms, so it has less of an effect on game balance. Some linnorms have DR 20/cold iron—another weak DR type, and again, their CR is somewhere around 20. Cthulhu (CR 30) has DR 20/epic and lawful. Demilich is a rare exception with DR 20/— at CR 14. I believe it's the kind of creature that was intentionally made hard to kill with physical attacks. At any rate, a creature shouldn't normally have DR 20/—, much less at CR 14.
DR 15 is similar to DR 20; DR/magic and DR/cold iron are the most common types; adamantine, good, epic and other types are possible. I could only find one creature with DR 15/—, the plasma ooze. It also has immunity to bludgeoning and piercing damage, so I believe that much like the demilich, the author made a point with the DR: fighting this creature with physical attacks won't be easy.
DR 10 has more variance in types. The most notable difference is that DR/— is more common. It's mostly just elementals, though, that have this type of DR. Much like DR 10, DR 5 has more variance in types than the bigger DR range. DR/epic is less common—only mythic creatures seem to have DR 5/epic.
DR 1, 3 and 4 are rare, while DR 2 is a bit more common. The most common type in this range DR/cold iron because many of the creatures are of the fey type. Above DR 5, the DR value is always divisible by five, no exceptions.