Terrain Dominance allows the HW to treat enemies native to that terrain as favored enemies, using the terrain bonus as the favored enemy bonus, correct?
So in effect, you need the environment the creature is native to from the bestiary? if you have Mountains +8, and run across a Troll, you get +8 to hit and damage, since they're native to Cold Mountians.
My question is, what about creatures that have "Any" as the native environment?
At 3rd level, a horizon walker learns total dominance over one terrain he has already selected for terrain mastery. When dealing with creatures native to that terrain, the horizon walker treats his favored terrain bonus for that terrain as a favored enemy bonus (as the ranger class feature) against those creatures. This bonus overlaps (does not stack with) bonuses gained when fighting a favored enemy.
Environment: The regions and climates in which the creature is typically encountered are listed here; these often present wider ranges than the icons at the top of the stat block indicate. In this case, the icon listed at the top of the stat block indicates the creature’s preferred terrain.
These are not all the same things. A native environment must necessarily be specific; you can't be native to everywhere (by the plain meaning of the word "native"). The "environment" stat line is just where you're likely to encounter them, not even what they prefer, let alone where they're from (unless it's specific).