Poisonous Blob: Grimer, Gulpin, and Yabakuron: Name explains it all. A pure poison type.
Although this has only been done once...
Cute Fairy: Clefairy and Minccino: A very cute pokemon. When it evolves, it retains most of it's previous features.
Psychic Fasionista: Jynx, Gardevoir, and Gochiruzeru: A Psychic type whose appearance is based on a certain type of Fashion.
Electric Mammal: Mareep, Shinx, and Blitzle: An electric mammal. Generally uses an aspect of it's real life animal counterpart to release electricity
Healer: Chansey, Miltank(?), and Tabbune: Pokemon that heal others.
Water dweller: Poliwag, Wooper(?), Surskit(?), and Omataro: Amphibious water dwelling pokemon.
The two warriors: Hitmonlee/Hitmonchan, Nageki/Dageki: Two fighting type pokemon that somewhat similar appearances and are in a way opposite in fighting style.
Emo canine: Lucario and Zoroark
Pokeballer: Voltorb and Tamegetake: Pokemon that have a spherical area on their design to have the pokeball design
Tentacles: Tentacool and Pururiru: Water type Jellyfish/Squid/Octopus
Angry pinecone: Pineco and Tesseed: A limbless pokemon with very little visible ways for it to attack it's opponent
Steel Duplicator:Magnemite and Klink
Cute-to-Ferocious Bear: Teddiursa and Kumasyun
A load of Bull: Tauros and Buffalon
Krab kings: Krabby, Corphish, and Ishizumai
Poor old fish:Magikarp, Feebas, and Shibishirasu
Ugly Duckling:Psyduck and Koaruhie
Cute Seal: Seel and Spheal
Cute Ghost:Gastly, Duskull, and Hitomoshi