The entire premise of Pokemon is that trainers "Gotta Catch 'Em All," but it is easier said than done.
There are hundreds of Pokemon now. Completing the current Pokedex not only takes some skill, but it takes a lot of time. Everyone Pokemon encounter isn't a guaranteed capture.
There are some Pokemon that seem impossible to catch. The likes of Caterpie and Rattata are sorely missed when squaring off against these creatures. Difficult is an understatement.
Top 5 Pokemon that are nearly impossible to catch
#5 - Abra
Nowadays, snagging an Abra isn't as impossible, but there was one point where it was one of the most evasive Pokemon. It is common knowledge that Pokemon often need their HP lowered in order to capture them.
Abra would almost always use Teleport and escape those battles. Either an early Poke Ball wouldn't work or the attack would not faint it and it would Teleport. Otherwise, the attack would faint it and players would have to start all over again.
#4 - Female Salandit
Female Salandit only make up about 12% of the Salandit population. They are necessary in order to evolve. A Salazzle will not come from a male Salandit. It will stay a Salandit no matter what level it reaches. The small portion of the population this Pokemon makes up leaves players often frustrated. It is extremely hard to find one, making it near impossible to catch.
#3 - Kangaskhan
In the original Pokemon games, Kangaskhan was not only hard to catch, but hard to even come by. It had about a 5% chance of appearing in the Safari Zone. Players had to pay for entrance into the Safari Zone and go in with Kangaskhan as the only goal.
The time limit was not the only detriment here. It is a stronger Pokemon and there is no way to lower its HP. More often than not, it would break out of the Poke Ball and the player would run out without a chance.
#2 - Mew
It is a lot easier to catch Mew these days, with the purchase of the Poke Ball plus playing one of the Let's Go games. It is a shame that one of the rarest Pokemon was made easily available. Still, many find it impossible to capture due to not having access to the aforementioned items.
In the Gen 1 games, Mew was available through events or cheating, that's it. If the glitch method was used, it would never be able to move onto other Generations as Pokemon Bank read it as broken.
#1 - Arceus
The Pokemon God is still extremely hard to come by. It was only available in Generation 4. Simply time traveling or cheating through walls was not enough to trigger an encounter. A special item was needed.
It could be obtained by an event, but in-game, players needed to summon it via hacks. Even then, it could take well over hundreds of Ultra Balls to catch it.