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I feel that the current list of Pokémon counterparts is woefully lacking. These are my proposed additions, with reasons as to why they should be included.
Wingull and Chinchou
- They share a catch rate of 190.
- They are both dual-typed Water Pokémon.
- Wingull is also part Flying and Chinchou is also part Electric, which is strong to Flying.
- They evolve two levels apart, Chinchou evolving at level 27 and Wingull at level 25.
- In addition, they both evolve only once.
- They both have 50/50 gender ratios.
- Their Almia Browser numbers are 10 apart (Wingull's is 78 and Chinchou's is 88).
- Their Dream World abilities, Rain Dish and Water Absorb, both recover HP when in the presence of water.
- However, the exact mechanisms behind the HP recovery are different.
Venonat and Diglett
- They were both introduced in Generation I, and the Diglett evolutionary line comes right after the Venonat line in the National Pokédex.
- They can both have up to three abilities. Interestingly, their abilities seem to correspond to one another.
- Venonat's Dream World ability, Run Away, negates one of Diglett's normal abilities, Arena Trap.
- Diglett's Sand Veil boosts its evasion, while Venonat's Compoundeyes boosts the corresponding stat, accuracy.
- Diglett's Dream World ability, Sand Force, raises the power of Rock, Ground, and Steel moves in a sandstorm; Venonat's Tinted Lens also raises the power of some moves depending on their type, although in this case it only applies to "not very effective" moves.
- Both species' eggs hatch after 21 egg cycles.
- Both species have 50/50 gender ratios.
- Both species' Red and Blue Pokédex entries referred to the Pokémon living in darkness: Diglett underground, and Venonat in the shadows of tall trees.
Pidgey and Spearow
Meowth and Skitty
Lillipup and Purrloin