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Couldn't we add or change out a Knowledge skill for the the correct enemy? Like Dean, the first level ranger, choose evil outsider as his favored enemy. Would you think he would exchange maybe Knowledge(nature) for (planes)?
I think Kenzer Co. had the most elegant option for this in their Kalamar setting. Knowledge (Monsters) that was available to Ranger and a couple other classes. I might suggest calling Knowledge (Fauna) so as to remove implications that it only applies to monstrous creatures.
graywulfe
Having a separate skill for monsters is a bad idea, particularly considering that the existing skills already cover this area. I know that doesn't actually answer the underlying question, but you could always try the Planar Ranger variant in Unearthed Arcana.