Shiny Marshadow Origin = Alcremie
Shiny Arceus Origin = Larvesta
Shiny Pikachu Origin = Eevee
Shiny Toxel Origin = Amped Toxtricity
Shiny Chikorita Origin = Cherry Blossoms
Shiny Bayleef Origin = Cherry Blossoms
Shiny Meganium Origin = Cherry Blossoms
Shiny Kabutops Origin = Skeleton Kabutops
Shiny Gastly Origin = Yin
Shiny Haunter Origin = Yang
Shiny Gengar Origin = Yin/Yang
Shiny Torchic Origin = Blue (da ba)
Shiny Combusken Origin = Blue (de)
Shiny Blaziken Origin = Blue (da ba)
Shiny Mega Blaziken Origin = Blue (di)
Shiny Gible Origin = Beta Gible
Shiny Gabite Origin = Beta Gabite
Shiny Garchomp Origin = Beta Garchomp
Shiny Mega Garchomp Origin = Beta Garchomp
Shiny Scyther Origin = Scizor
Shiny Scizor Origin = [n/a]
Shiny Mega Scizor Origin = [n/a]
Shiny Ho-Oh Origin = Rainbow (originally just Lugia inspired)
Shiny Lugia Origin = Ho-Oh
Shiny Celebi Origin = Gold, Silver and Crystal

If you're wondering why Redesigned Shiny Marshadow is based off of Alcremie and why Redesigned Shiny Arceus is based off of Larvesta...

Do I need to say more?