List of Amorphous Pokémon

Generation 1

• Grimer & Muk
• Gastly, Haunter & Gengar
• Koffing & Weezing

Generation 2

• Misdreavus & Mismagius
• Wobbuffet
• Slugma & Magcargo

Generation 3

• Ralts, Kirlia, Gardevoir & Gallade
• Gulpin & Swalot
• Castform
• Shuppet & Banette
• Duskull, Dusclops & Dusknoir
• Chimecho (If you want, you can count Chingling)

Generation 4

• Shellos & Gastrodon
• Drifloon & Drifblim
• Spiritomb
• Rotom

Generation 5

• Yamask & Cofagrigus
• Solosis, Duosion & Reuniclus
• Frillish & Jellicent
• Tynamo, Eelektrik & Eelektross
• Litwick, Lampent & Chandelure
• Stunfisk (Yes, this Pokémon is here too)

That's it for now.