"Gotta Catch 'Em All" is more than just a slogan for many Pokemon fans. While some are content wrapping up the game when they become the champion and see the credits, others have an unbreakable determination to fill their Pokedex and see every Pokemon the world has to offer.

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Naturally, that isn't always an easy task. Strong Pokemon, such as Legendaries, have very low catch rates, but a knowledgable player will still be able to get their hands on them fairly easily. The Pokemon that are real pains are the ones with ridiculously low encounter rates, or a tedious series of tasks required to get to them in the first place. Not counting event-only Pokemon, here are the hardest ones to obtain from each generation of games.

8 Generation 1: Game Corner Porygon

Gen I Pokémon

With just 150 Pokemon needed to fill the Pokedex (aside from an event to get Mew), Generation 1 doesn't have many annoying Pokemon to catch. Some 1% Tall Grass encounters are a bit annoying, but they're not that big of a deal in the grand scheme of things.

Porygon takes the crown for this generation because there's only place to get it, and doing so takes a lot of boring busywork. The Celadon City Game Corner is the only place Porygon is available, and players must buy it with coins that they get either through winning slots or buying the coins outright. Even buying them takes a while, as Porygon is the most expensive Pokemon (6500 coins in Pokemon Blue, 9999 in Pokemon Red) and coins can only be bought 50 at a time. It's not difficult, but it's a grind.

7 Generation 2: Roaming Entei, Suicune & Raikou

Gen 2 Pokémon Legendary Beasts

Generation 2 boasts a triple feature, because all three of these Legendary Beasts must be found the same way. The words strike dread into the heart of any Pokedex completionist: Roaming Pokemon. These Pokemon randomly move around the world map, teleporting miles whenever players use Fly to fast-travel.

Players must chase these Pokemon down using the world map until they're lucky enough to find themselves on the same route as one of them. Then, when they finally encounter them, they will be in for a shock if they aren't ready. These Pokemon will always attempt to flee the battle if they can, forcing players to go hunting for them again. Trainers must come prepared with a way to prevent their escape.

6 Generation 3: Fishing Feebas

Pokemon Feebas

A name cursed by many fans of the older Generations, Feebas is notorious for its elusiveness in Generations 3 through 5. Each one makes it annoying to catch in their own way, but Generation 3 takes the cake for causing the most frustration.

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In Gen 3, Feebas can be found on Route 119 by fishing in the water, but there's a catch. There are hundreds of water tiles on Route 119, and Feebas can only be found in a paltry six of them. What's even worse is that, in those six tiles, Feebas isn't a 100% encounter rate, meaning players will need to check each tile multiple times to be sure. Even then, the laws of probability could still screw trainers over.

5 Generation 4: Honey Tree Munchlax

A Munchlax eating

Not content with the horror of Generation 3's Feebas, Game Freak upped the stakes in Generation 4 and made arguably the single hardest Pokemon to find in the entire franchise: Honey Tree Munchlax. There are lots of honey trees scattered across Sinnoh. Players can spread some Sweet Honey on these trees, come back six hours later and find a rare Pokemon.

A couple factors make Munchlax so much worse than other honey tree Pokemon. Firstly, it only has a 1% encounter rate, which is a pain when players have to wait 6 hours between each encounter. Secondly, out of the many trees across Sinnoh, only four can actually spawn a Munchlax, which are randomly assigned when a new save file is generated. There is a way to locate two of them, but the other two are based on a secret value that players have no way of learning.

4 Generation 5: Black & White Zorua

Although Zorua has become much more easily accessible in the Generations since, in its debut, it was treated like a Mythical Pokemon, and was similarly difficult to obtain. in Pokemon Black & White, it can be obtained as a gift Pokemon, but there is a big caveat about obtaining that gift.

Firstly, players must have Celebi in their party, a Mythical Pokemon that could only be obtained through events. However, that wasn't enough. It has to be a specific "Fateful Encounter" Celebi given away in a 2011 event. Thankfully, it can be retrieved without the event if the player knows where to go and what password to enter into the game. Taking that Celebi to a lost girl in Castelia City is the only way to obtain Zorua.

3 Generation 6: Roaming Articuno, Zapdos & Moltres

Generation 6 has a reputation in the Pokemon fanbase for being easier than most other Generations, and when it comes to completing the Pokedex that holds true. There aren't any ridiculously hard Pokemon to locate like in the previous few games. That said, a few still require legwork.

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Only one of the Legendary Birds is catchable in an X/Y save file, decided by which Starter Pokemon the player chose. After defeating the Elite Four, players will randomly encounter their designated bird in the grass. It will automatically flee without a chance for battle. Players must track it with the Pokedex to find it again...and again...and again. However, they will never be able to battle it in these encounters. Players must repeat this process twelve times before the bird finally settles in Sea Spirit's Den for a battle.

2 Generation 7: 100% Zygarde

Pokemon Zygarde

Despite being a Generation 6 Pokemon, Zygarde didn't get a proper feature until Pokemon Sun & Moon in Generation 7. It had to take a bit of a back seat in the story-heavy world of Alola, but it finally got its extra 10% and 100% forms.

Scattered around every nook and cranny of Alola are Zygarde cells. When the player finds one, it will absorb into their Zygarde cube. Taking this cube to the research centre on Route 16 allows enough of the cells to form into a Zygarde. 10 Cells gets the 10% form, 50 Cells gets the standard 50% form. However, finding all 100 cells is the only way to get the 100% form. This form starts with the same appearance and stats as the 50% form, but once it drops below half health, it will transform into an astoundingly powerful Pokemon.

1 Generation 8: Crown Tundra Keldeo

Despite normally being reserved for events, Game Freak has occasionally surprised players with a legitimately catchable Mythical Pokemon in the main games. Deoxys was catchable in a surprise battle at the end of Pokemon Omega Ruby & Alpha Sapphire, but that looked like it would be all until the Crown Tundra DLC released for Pokemon Sword & Shield.

Around the area, players could find mysterious footprints. There were three different sets around the Crown Tundra, and finding enough of each would cause Cobalion, Terrakion or Virizion to spawn in the world. This was a cool quest for players, but there was an unexpected surprise at the end. Once all three Swords of Justice had been caught, players could go to a little island tucked away in the Crown Tundra where, shockingly, Keldeo would spawn as a catchable Pokemon.

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