Apart from the obvious, like Chikorita/Cyndaquil/Totodile being the equivalent of Bulbasaur/Charmander/Squirtle respectively, I can see some similarities in each Generation. I might be wrong though, but as I said it’s just my impression!
1) Caterpie & Metapod & Butterfree / Ledyba & Ledian
One of the earliest Pokemon of their respective generation which becomes much stronger when it reaches the last Evolution Level, but in Ledyba’s case it skips the chrysalis phase.
2) Ditto/ Smeargle
The copycat Pokemon. Just like Ditto transforms into its opponent Pokemon, Smeargle copies its opponent’s moves.
3) Dratini & Dragonair & Dragonite / Larvitar & Pupitar & Tyranitar
The last Trio before the final Legendary Pokemon of each generation. Also considered the strongest Pokemon once it reaches its final Evolution Level.
4) Farfetch’d / Murkrow
The outsider bird Pokemon of each generation. No evolution form and not strong enough compared to other stand out Pokemon, like Lapras or Corsola.
5) Fearow / Ho-Oh
Ho-Oh kinda sticks out actually, among the legendary Pokemon of the Generation 2 if you notice as well. Entei, Raikou and Suicune are the equivalent of Moltres, Zapdos and Articuno , Lugia is the equivalent of Mewtwo and Celebi is the equivalent of Mew. But what about Ho-Oh? I believe that normally it would be Level 2 Evolution of Fearow, by giving it a Rainbow Wing in order to evolve, but it became a Legendary Pokemon just because it was supposed to be the mascot of Gold Version in the old Pokemon games, opposed to Silver.
6) Grimer / Slugma
Like Grimer is a walking mass of Acid, Slugma is a walking mass of Lava.
7) Growlith / Houndour
It’s pretty much obvious that Houndour was heavily inspired by Growlith. Also Growlith has some Dark elements as a Pokemon, but Dark type didn’t exist when first Generation came out, so it remained just a Fire type Pokemon.
8. Lapras / Mantine
Big sized sea Pokemon, which also appears to be friendly.
9) Magikarp / Tyrogue
Both have the When I evolve I am going to kick your butt syndrome. Tyrogue is a milder form of Magikarp though, since it’s stronger than Magikarp, and Hitmonlee/Hitmonchan/Hitmontop are weaker than Gyarados. Tyrogue might share some resemblances with Eevee as well, since it has multiple Evolution forms too.
10) Magnemite / Unown
Sure entirely different with each other, but I believe that the idea of creating Unown was based on Magnemite. Take for example the concept with the one eye. Have you ever thought that Magnemite could have different letters as its side magnets? Personally I had!
11) Oddish & Gloom & Villeplume / Hoppip & Skiploom & Jumpluff
Let’s say that this trio is a happier version of Oddish and co. While Oddish and its evolution forms is darker and more gloomy –as the name of its Level 1 evolution form suggests too- Hoppip (Hop-pip) and co. is the upbeat version of its Generation 1 pals.
12) Paras & Parasect / Spinarak & Ariados
Pretty similar in terms of being Bug type, and also having one evolution form each, and also their shape is pretty similar, especially Paras and Spinarak.
13) Pidgey / Hoothoot
The annoying bird type Pokemon that is almost everywhere. Hoothoot not in the same degree as Pidgey though.
14) Pikachu / Togepi
The fan favorite Pokemon. Sure, Togepi wasn’t the phenomenon Pikachu became, but definitely it was the mascot of Generation 2.
15) Pinsir / Heracross
Pinsir normally would also be a Fighting Pokemon I believe, but anyway both are Bug-type Pokemon, who are able to pick up a fight with the likes of Hitmonlee or Machop.
16) Psyduck / Wobbuffet
The stupidest (well, according to the TV series) Pokemon of each generation.
17) Rattata / Sentret
Whether we talk about old Pokemon games or Pokemon GO, this is the most frequent Pokemon of each generation.
18. Snorlax / Sudowoodo
The Pokemon that blocks your way at some point of the game. Sure they look way different but they serve the same purpose on the game.
19) Tauros / Miltank
A bull and a cow. So obvious eh?
20) Voltorb & Electrode / Pineco & Foretress
Voltorb/Electrode were Pokemon in a Pokeball disguise somewhat. I believe Pineco/Foretress were based on the same concept, but this time they were disguised in a pinecone (in Pineco’s case) and a…well I don’t know what Foretress is exactly, but sure it’s based on Electrode.