Question

Changing of forms does not count.

• Scyther -> Scizor: Speed drop
• Magneton -> Magnezone: Speed drop

Comments

I was just about to ask this question! Onix's speed decreases when it evolves into Steelix, so does Electabuzz's speed.

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Trapinch has an Atk of 100, but Vibrava has 70 Atk. It goes back to 100 as Flygon.

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Trapinch's attack falls when it evolves into Vibrava.

Answer 1

Caterpie -> Metapod: Speed
Metapod -> Butterfree: Def
Weedle -> Kakuna: Speed
Kakuna -> Beedrill: Def
Poliwhirl -> Poliwrath: Speed
Poliwhirl -> Politoed: Speed
Magneton -> Magnezone: Speed
Onix -> Steelix: Speed
Scyther -> Scizor: Speed
Porygon2 -> Porygon-z: Def & SDef
Murkrow -> Honchkrow: Speed
Wurmple -> Sil/Cascoon: Speed
Silcoon -> Beautifly: Def
Surskit -> Masquerain: Speed
Nincada -> Ninjask: Def
Nincada -> Shedinja: HP & Def
Trapinch -> Vibrava: Atk
Shelgon -> Salamence: Def
Combee -> Vespiquen: Speed
Tangela -> Tangrowth: Speed
Roggenrola -> Boldore: Speed
Swadloon -> Leavanny: Def SDef
Dwebble -> Crustle: Speed
Karrablast -> Escavalier: Speed
Shelmet -> Accelgor: Def & SDef
Larvesta -> Volcarona: Atk

I think I have every stat drops here. It has taken me long time, but here you have it!
Edit: Electabuzz -> Electivire: Speed & Magmar -> Magmotar: Speed

Comments

You've missed a few. There is no Magmar, no Electabuzz.

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Thanks :)