Here's a question: if you were DMing an Adventure Path and you saw something like this, would you think it's stupid or think it's kind of neat:
Iron Golem
Note: The golem also has a musket that it can utilize, which it does so if no one is within reach. The same magic animating the golem enchants the musket with unlimited ammunition. The gun has no chance of misfiring.
+13/+8/+3/-2 ranged touch (2d8+10/x4)
Feats: Deadly Aim
Basically, typical Iron Golem as base stats, but a little different from normal. Or:
Elder Fire Elemental
Note: This elemental is healed by negative energy and injured by positive energy (as if it were undead). In addition, it half the burn damage listed is negative energy rather than fire. When it dies, it releases the negative energy in a blast of power, dealing 7d6 negative energy damage in a 30ft radius (Will save DC 22 for half).
I've noticed that I've been doing this more recently, adding slight changes rather than feeling the need to create brand new creatures. And yes, there are reasons for this, neither variant is arbitrary. Now, if stats change or if I was doing something grander, of course I would just stat up the creature, but as you can see, neither option drastically changes the creature enough to completely rewrite the statblock.