Of all the things that stick out in the metagame of Pokémon's first generation, the lack of a hard counter to psychic-type was most glaring. Alakazam was top dog and nothing could stand up to its dominance. So when Gold and Silver (and eventually Crystal) came out, the metagame was shook to the core as the world seemingly changed around Alakazam. Thus Gold and Silver signaled the beginning of the end for Alakazam's glory days.
Second Generation: The First Great Split
So many changes in GSC were subtle yet important changes that helped shape how our current metagame is today. New types were introduced, new moves were introduced, old moves were retooled to be useful, and certain items could be carried for strategic use.
One thing new trainers take for granted are the two stats of special attack and special defense. A separate stat for special attacks and another for defense of those special attacks? But for Gold and Silver, this change alone caused legends to crumble and let new ones take their place.
Suddenly, Alakazam wasn't as defensive as it used to be as its special defense was mediocre at best. Likewise, Snorlax, who typically lived in Alakazam's shadow, got boosted by an incredible special defense.
"Yeah, I knew a guy named Snorlax. I hear he's running my turf now."
But the special split isn't the biggest reason why Alakazam tumbled out of grace. In an attempt to introduce a predator to something that didn't have any natural predators, Game Freak added two new types, both which resisted psychic: steel and dark-types.
Steel-types immediately became known as the premier defensive type. With natural resistances to 11 out of the 17 types before accounting for a secondary type, steel-types were seen as a very capable choice to switch into when you needed to stall for time.
Dark-types on the other hand, were introduced as a true hard counter to psychic-types. Not only were dark attacks strong against psychic-types, dark Pokemon were completely immune to psychic attacks!
Not only did the introduction of these two types hinted at the downfall of psychics, it also increased the usage of fighting-types as both steel and dark coincidentally were weak to fighting.
Hope you like fighting-types, because more are on the way!
One of the most important new moves introduced this generation was Curse, which boosted attack and defense one stage and decreased speed by one stage. Nearly every bulky attacker carried this baby and it defined the general pace of competitive battling. Fights took longer as major threats like Snorlax, Tyranitar, and Steelix abused their bulkiness to sponge hits even better, then attack with the force of a truck with the help of Curse. Snorlax alone gave birth to a moveset forever dubbed as Curselax. With it's high special defense and mediocre defense being boosted by Curse, Snorlax became nigh unkillable and unstoppable after a few turns of Curse.
Curse changed the simple back and forth game of type matching and slowed it way down. Defense and stall tactics were born as many Pokemon this generation were blessed with powerful defenses. Curse isn't the only example of the slowed pace though. Sleep Talk was introduced which picked a move at random when your Pokemon was asleep. This led to the Rest/Sleep Talk combo, allowing Pokemon to recover health and continue attacking. Roar was also given a new function in forcing trainers to switch, thereby losing any boosts they've gathered (sometimes the only way to stave off the threat of a Snorlax attempting to sweep). The metagame became focused on taking hits and returning the damage with interest. Case in point, another infamous name: SkarmBliss.
Skarmory was the bird king of physical defense. As a steel-type, it already had great resistances. And even though steel was weak to fighting, it's secondary flying type negated its fighting weakness. It also made it immune to everyone's favorite catch all physical attack, Earthquake.
So you decide to throw in Starmie in an attempt to Thunderbolt Skarmory? Meet the other end of SkarmBliss: Blissey. The newly introduced evolution of the already specially defensive Chansey, Blissey has been and still is the best special wall in the game. Trainers go to great lengths to make sure Blissey gets taken out to make their life much easier. Nicknames for Blissey have ranged from fat, pink blob to asshole.
Lastly was the introduction of held items. Certain, simple items could be used by Pokemon during battle. Pokémon couldn't hold and use something man-made like a potion, but they could hold a berry which they could eat mid-battle for a burst of health. This also furthered the prevailing strategy of defense with another thing that would stay relevant for years: Leftovers. When held, it allows the Pokemon to recover a fraction of their health after every turn. So once Snorlax used Curse enough times, not only did the damage being done to it become negligible, but the fractional healing of Leftovers could overtake any damage you did to it. Almost every Pokemon has a moveset that plans for the use of Leftovers. It's still a popular item today and it's probably why the item clause was created for competitive battling.
All of the things listed above led to much, much longer lasting games. The battles of yesteryear, which could have been decided within 5 or 10 minutes, now became 30 or 60 minute wars of attrition.
Alakazam took a hit in power but held on while Snorlax became everyone's favorite. Starmie, Gengar, and Rhydon held firm to their positions. As mentioned before, Chansey remained though in the new form of Blissey, while Skarmory became a new threat as a physical wall opposite of Blissey.
The new threats introduced were a testament to the new types. Steel-types like Steelix and Forrestress became popular for being general purpose defense. Forretress was also one of the few Pokemon to learn a new move which created the idea for entry hazards: Spikes. Set down Spikes and watch as your opponent is helpless to damage on the switch. Once Forretress got all his Spikes down, it's just a matter of using Explode and sending in your next big threat.
Dark-types like Tyranitar and Houndoom were not only threatening to psychic-types but also carried a powerful dark-type move. Pursuit allows the attacker to hit Pokémon who are being switched out before they completely retreat, creating trap situations. If something like Alakazam stays in against Tyranitar, Pursuit will be enough to kill it. However, switching out will still get it killed as Pursuit will hit on its way out.
The metagame in GSC slowed to a crawl with the advent of things like Rest/Sleep Talk, Toxic, Curse, and Leftovers in tandem with the newly introduced steel-type. While still enjoyable, it was probably for the best how the metagame changed and slowly became more and more fast paced in future generations.
Next time, we move into Ruby and Sapphire. Battling becomes more diverse with the introduction of abilities, more complex with natures, and the forecast calls for crazy weather!
Research and credit goes towards the archives of Smogon. Image credits to Smogon and Bulbapedia.