How can I convert monsters from Pathfinder to 3.5e?

Question

The Wikipedia article says that Pathfinder is intended to be backward-compatible with 3.5e:

It extends and modifies the Revised 3rd edition Dungeons & Dragons (D&D) game rules published by Wizards of the Coast under the Open Game License. Pathfinder RPG is intended to be backward-compatible with Dungeons & Dragons edition 3.5.

I assume "backward-compatible" means I can take 3.5e content and use it in a PF campaign. But what about vice versa? What I've found so far is that 3.5e monsters with the same name and CR tends to have lower stats in PF. For instance, see Ankheg (PF/D&D), Dire Rat (PF/D&D) or Owlbear (PF/D&D).

Can I use monsters from the Pathfinder SRD in a 3.5e campaign without changing their Challenge Rating, or does it cause balancing issues by making encounters too easy?

Answer 1

Yes, you can, though you will have to sort out their skills in some cases, if they are relevant. Since Pathfinder combined several 3.5e skills, Pathfinder creatures with bonuses to the Pathfinder skills may have bonuses to one or all of the 3.5e skills that were combined into the Pathfinder skill.

You also have to calculate the grapple bonus if you really want to recreate a 3.5e-style monster stat block. In practice, you probably only need to do that for monsters that are going to be grappling, or if you have a PC that does a lot of grappling.

But then we get to the issue of CR. Will the Pathfinder CR be “accurate” in 3.5e? No—because the Pathfinder CR isn’t accurate even in Pathfinder. Nor are 3.5e CRs accurate in 3.5e. CR is a wildly-varying mess that is at best a first-pass approximation of the monster’s actual challenge to your actual party. Both systems absolutely require manual comparison of the monster’s abilities compared to the party’s abilities in order to determine how challenging it is likely to be. Blindly using CR will not lead to much success in either system.

Compared to all the problems that CR already has, the difference between Pathfinder CRs and 3.5e CRs is, in my experience, negligible. There may be some outliers that are particularly mis-CRed when ported to another system—but both systems have outliers that are wildly mis-CRed without any porting, so meh.