With the release of Sword and Shield on the horizon, fans are revving up to catch some brand new Pokémon. Battling and befriending these little monsters are the core of the series, but sometimes it's very hard to convince them to join your squad. Some of these 'mons are just very independent.

RELATED: Pokémon: The Toughest Gen I Gym Leaders, Ranked

Trainers have been chucking Pokéball after Pokéball at some of these creatures since the very beginning, back in good old Kanto. Some of these Pokémon are very powerful or have become fan favorites, but others may not be worth the effort required to catch them. Let's look at some of the most difficult Kanto Pokémon to catch!

10 10. Abra

One of the Kanto region's few Psychic-type Pokémon, Abra would have been an incredible catch for any enterprising young trainers. Psychic Pokémon in the early games were almost unrivaled in their dominance and Abra, after some training and trading, evolves in Alakazam, one of the strongest of its type. However, catching an Abra was no easy feat. Unlike most of the Pokémon on this list, Abra doesn't have an absurdly low catch rate. It just has the annoying habit of teleporting out of battle on its first turn, turning every encounter into the spin of a roulette wheel.

9 9. Single-stage Evolutions

Almost every single-stage evolution in the Kanto region has the exact same catch rate of 45. In addition to the three above (Electabuzz, Mr. Mime, and Magmar) this includes Pokémon like Farfetch'd, Onix, Hitmonlee and -chan, Lickitung, Tangela, Kangaskhan, Scyther, Jynx, Pinsir, Tauros, and Lapras.

RELATED: Pokémon: The 10 Most Useful Poké Balls, Ranked

Usually the more powerful Pokémon, like third-stage evolutions, have a pretty low catch rate in the wild; however, it's easy to find the earlier evolutions and then train them up to their most powerful form. These Pokémon don't have the luxury of weaker earlier forms (at least, not in Generation I) to hunt down, making every capture feel special.

8 8. Eevee

The Pokémon franchise's secondary mascot has never been particularly easy to catch. Finding an Eevee is supposed to be a special occasion, let alone catching one. Even their gender ratio is heavily skewed male to prevent most players from breeding them into infinity. They're unique additions to any team thanks to their ability to evolve into many different Pokémon based on their environment. It almost seems like cheating when you're handed one right out of the gate in Pokémon: Let's Go, Eevee! Back in the day we had to work for this little fluffball!

7 7. Dratini

This one probably comes as no surprise, considering Dratini and its evolutions form the very first psuedo-legendary Dragon-type line. And in fact the only Dragon-type Pokémon in the entire first generation. So snagging one was pretty difficult based on it's catch rate alone, but you also had to find one. In Generation I, they only appeared in the wild in the Safari Zone, and only if you were using a Super Rod. Of course, if gambling's your thing, you could also buy one from the Game Corner with enough coins.

6 6. Ditto

Ditto has always been and will always be one of the strangest ideas for a Pokémon. It's nothing more than a pile of pink goo, but it has one of the most versatile abilities in the entire franchise. It can assume the form, including the moves, of opposing Pokémon and use your opponent's strength against them.

RELATED: Pokémon: Every Villainous Team, Ranked According To Strength

This powerful ability is guarded by Ditto's low catch rate of 35, the only Pokémon to have such a rating. In later generations, Game Freak took pity on poor trainers and allowed Ditto to assume the catch rate of the Pokémon it was imitating instead.

5 5. Wigglytuff

Wigglytuff's catch rate isn't the lowest on this list. It's sort-of counterpart, Clefable, actually has a much lower catch rate of less than 8%. The only reason it's Wigglytuff on the list instead is that...you can't actually catch a Clefable in the wild. You can only acquire one by evolving a Clefairy with a Moonstone. Wigglytuff, on the other hand, can be found in exactly one place in the wild—the late game location of Cerulean Cave. So if you haven't been able to catch and evolve its prevolution, Jigglypuff, but you really want a Wigglytuff on your team, you're gonna need to start searching.

4 4. Chansey

Chansey is known for its massive HP stat, which makes weakening a wild one for catching a difficult proposition. These beefy boys can have up to 700 HP, as well as several moves that boost their defense or regain health. In addition to that difficulty, they only appear in the Safari Zone and in Cerulean Cave. And then they have a catch rate of less than 10%. Nurse Joys must have the patience of a saint to catch their little helpers. Better stock up on Pokéballs.

3 3. Snorlax

Snorlaxes in the world of Pokémon are a limited resource. You encounter one, maybe two, in an entire game if they're available in the wild at all. They're living obstacles, and you get one chance to go toe to toe with this wall of a 'mon before it flees from your life forever. Sure, you can save scum until you get lucky, but its limited availability and extremely low catch rate are sure to keep you restarting for awhile. Maybe it's better to just let sleeping bears lie.

2 2. Legendary Birds

It probably doesn't shock anyone familiar with the franchise that the Legendary Pokémon are among the hardest to catch in any given game. They're often spoken about like they're equivalent to gods, so it would make sense that it would be difficult to weaken them, let alone bend them to your will.

RELATED: Pokémon: Every Shiny Legendary Form Change, Ranked

You can find the three legendary birds of the Kanto region in set locations across the map: Articuno in the Seafoam Islands, Zapdos in the abandoned power plant, and Moltres on Victory Road. You should stock up on Ultra Balls, since their catch rate is a measly 3, or 1.6%. And you might want to hang on to your Master Ball...

1 1. Mewtwo

The ultimate prize for any Kanto trainer is the legendary Mewtwo. Created from the modified DNA from the exceedingly rare Mew, Mewtwo is one of the most powerful Pokémon that exists. And you want to make it your very best friend. First you need to enter the Hall of Fame, by defeating the Elite Four and the Champion. Then you have to make it through to the deepest part of Cerulean Cave, which is filled with high level Pokémon. Then you actually have to battle Mewtwo. Arceus help you if you've already spent your Master Ball, because this is a battle you want to end quickly.

NEXT: 9 Pokémon Cut From Sword And Shield That We'll Miss The Most