Since I heard that there was a power creep in Gen 7 OU, I was wondering: What is a power creep, how do they start, how can a pokemon in a tier undergoing power creep get affected by it, and weirdly enough, is it possible for a mon to actually benefit from a power creep?
Power Creep is basically when stuff generally gets stronger and as such the meta is defined by a new, heightened power level. This was extremely prominent in Gen 5 where we were introduced to a bunch of stuff with incredible Hidden Abilities like Drizzle Politoed/Drought Ninetales (which were so centralizing on their own that entire abilities needed to be banned because of how easily these two enabled them), Poison Heal Gliscor, Speed Boost Blaziken (which was so good it got banned), Rough Skin Garchomp (tbh this was Power Creep for OU because it actually nerfed Chomp enough to legalize it in OU in the first place), etc., as well as new heavy-hitters like Excadrill and Hydreigon offensively and Ferrothorn on the defensive end. The power levels rose to the point where OU staples in Gen 4 ended up dropping to UU, either because they couldn't do their jobs quite as well as they used to or were outclassed by stuff that could do their jobs better. That's a big reason why Gen 5 UU really closely resembled Gen 4 OU on many fronts: a lot of the stuff that was good in Gen 4 OU was much worse in the more hostile Gen 5 OU environment and as such received less usage.
This is prominent in Gen 6 and especially Gen 7 because of Megas and Z-moves as well as new additions like the Ultra Beasts and the Tapus that have extremely min-maxed stats and can perform specific roles so well that they warp the metagame around them. Pheromosa, during its brief stay in OU, had an enormous amount of offensive potential on both ends of the spectrum, packed Ice Beam for Landorus-T and Zygarde, and had a near-uncontested Speed tier and only had pitiful bulk which let it be prone to revenge killing and tended to limit its Quiver Dance opportunities (though it could still set up on a lot of passive threats). Tapu Lele's Psychic Terrain basically removes priority from the equation for a few turns, which gave Pheromosa that much more time to wreck house against offensive teams. Lele itself was extremely dangerous at the time and further enabled terrifying wallbreakers like Mega Metagross to shatter stuff, since Psychic Terrain's buffs could turn Zen Headbutt 2HKOs into OHKOs and 3HKOs into 2HKOs. This helped give rise to extremely difficult-to-wall offensive cores that used Tapu Lele as a key player in limiting their counterplay, though as it turned out Mega Metagross and Pheromosa were far more problematic on their own than Lele ever was due to their versatility and generally amazing stats and as such were banned.
One of the weirdest examples of Power Creep is probably present in Eviolite Chansey vs. Blissey; once Chansey got Eviolite it was generally bulkier than Blissey and could fulfill the most defensive roles better, while Blissey had to do things outside the box to better differentiate itself from Chansey during its final stint in OU (Calm Mind, something that even remotely resembles an offensive presence on the Special front, etc.). Nowadays Blissey is a UU Stall staple but sees no use in OU because Chansey is literally better. Another interesting example of Power Creep is present in Latios, who went from being Uber in Gen 4 to being one of the Gen 5 OU staples to being quite solid in Gen 6 to being completely and utterly unviable in OU this gen. Its Speed tier isn't what it used to be when stuff is faster than it. Its offensive prowess isn't what it used to be when some stuff is hitting harder while other stuff is bulkier. It's most certainly too strong for UU still, but that's because UU isn't really ready for its levels of power quite yet when Latias already runs a large part of the tier and Latios in that tier is just strictly better.
Power Creep can benefit stuff that is kitted to deal with specialized threats of that nature. One of the most evident examples of Power Creep benefitting something that already existed is present in
Tangrowth. In Gens 4 and 5 it was certainly usable but it was more prominent in lower tiers and received much less usage in OU. In Gen 6 Assault Vest became a thing and Assault Vest+Regenerator is a disgustingly good combination as it did a great job at patching up Tangrowth's one truly exploitable weakness and as such it saw some OU use and even rose to OU eventually. In Gen 7 there were more things that Tangrowth could naturally check alongside more things that it could counter with Assault Vest sets, such as Tapu Bulu, pre-ban Zygarde, Kartana, and Ash-Greninja: as a result Tangrowth was one of the most splashable, spammable defensive threats in the entire tier for a time because of just how many new additions to the tier it could beat. Tangrowth combined its amazing physical bulk, coveted and rare Ground and Thousand Arrows resistances, great mixed attacking stats, and excellent attacking movepool with new tools to patch up its lacking Special Defense stat to act as a blanket check to a bunch of the tier's most centralizing forces and it was so good at this role that it rose all the way to A+ on the VR and was on a pretty large number of teams because of how consistent it was at keeping these things in check. Now it's on somewhat of a decline, but it's unlikely Tangrowth will drop to UU this generation because it still has the tools to bounce back from almost any metagame shift. Power Creep would
negatively impact Tangrowth if we received a large influx of threats in a later generation that would be able to break through its defenses and hinder its legendary tanking capabilities. Stuff that would be able to more easily 2HKO even Assault Vest
and Physically Defensive Tangrowth from full HP, or stuff that would severely hinder its tanking capabilities by just outliving it and its teammates, would contribute to Tangrowth's decline and potentially warrant low enough OU usage that it would drop to UU, and its fate in a Gen 8 or 9 UU would be unclear since it could either be
way too good at walling stuff and would be sent to rot in UUBL, the power levels would be just right for Tangrowth to receive a healthy amount of usage, or the power levels would be too much for Tangrowth and it would drop into RU where it can fall under these categories again.
TL;DR: Power Creep is when new stuff does better than old stuff and old stuff falls out of favor, it usually starts at the beginning of a new generation or when new Move Tutor options are added, a mon can be largely unaffected/can improve or adapt/get significantly worse as a result of it, and there are a select few cases like with the aforementioned Tangrowth and Avalugg where Power Creep can turn the old stuff into the gold stuff without directly buffing the mons themselves.
Not saying Avalugg is the gold standard of what a mon should be, by the way. It has many, many, many,
many issues. But it's definitely a hell of a lot better in OU than it was last generation.