Horizon Walker is a core Prestige Class in 3rd Edition Dungeons and Dragons. It's seemingly easy to qualify for, requiring only 8 ranks in Knowledge (Geography) and the Endurance feat. Despite the very modest requirements, Knowledge (Geography) is actually a relatively rare skill on martial classes and in core only characters with at least one level in Ranger, Bard, Cleric with Knowledge domain, or Wizard can qualify "on time", and only Ranger remotely wants to enter it.
Entry requirements aside, it's actually a worthwhile class to enter, especially in core only games. The first five levels give a "Terrain Mastery", which lets you select a terrain then gain both a bonus to a relevant skill and a small bonus to attack monsters native to that terrain. The two exceptions to this are Desert (which gives immunity to fatigue and is amazing for Barbarian) and Underground (which gives Darkvision, which is nice) don't give skill bonuses. One easily overlooked part of this feature is that the bonus against monsters is not based on where you are, but the location section of their monster entry. This section disproportionately favors certain terrains over others so some picks apply to a lot of monsters. At sixth level till 10th, you instead gain a "planar terrain mastery" which gives unique and worthwhile abilities depending on the plane chosen. Of particular note is the Shifting plane option, which gives a Dimension Door SLA every 1d4 rounds. That's a fantastic ability to be able to spam and worth taking six level. Overall Horizon Walker is surprisingly well designed for a martial prestige class in core.
Pathfinder, in its mission to kill all non-dual advancement prestige classes, badly nerfed Horizon Walker to the point it would only ever be worth a two level dip for fatigue immunity except for one build. Terrain Mastery no longer gives a bonus against monsters, and Shifting (now Astral) only works 3+wis mod times a day. In return, it gains a bunch of instances of the Ranger's meh Favored Terrain feature and Terrain Dominance, which grants the ability to treat creatures native to a terrain as favored enemy for a limited number of favored terrains and some random SLAs. The single remaining use for Horizon Walker is to use a Rogue talent to get Favored Terrain to an absurd bonus, then use the Ranger spell Instant Enemy to get that absurd favored enemy bonus against anything.
An archetype that any class can take. You become well adapted to a single environment your GM decided to set the campaign in. Gains bonuses and ignore hazards while traveling through that Forest, Desert, Ocean, or other places.