Which Fateful Encounter Pokemon unlock which events?

Question

This question is for all Pokemon games.

Answer 1

Shaymin
Pt: Gracidea obtainable in Floaroma Town Pokemon Center
HGSS: Gracidea obtainable in Goldenrod City Flower Shop
BW: Gracidea obtainable in Lacunosa Town Pokemon Center
B2W2: Gracidea obtainable in Striaton City Pokemon Center

Regigigas
Pt: Makes Rock Peak, Iceberg, and Iron Ruins available

Shiny Pichu
HGSS: Activates an encounter with Spiky-eared Pichu in Ilex Forest

Arceus
HGSS: Allows player to teleport from the Ruins of Alph to the Sinjoh Ruins

Celebi
HGSS: Initiates a battle with Giovanni at Tohjo Falls
BW: Allows player to obtain Zorua in Castelia City

Shiny Entei/Raikou/Suicune
BW: Initiates a battle with Zoroark in Lostlorn Forest

Events triggered by specific fateful encounter Pokémon (Bulbapedia)

Answer 2

In HG/SS Arceus unlocks Shinjo Ruins (may be done 2 times, one with event Arceus, one with Azure Flute Arceus) where you can get Dialga/Palkia/Giratina at lv.1.
Shiny beasts (Entei/Raikou/Suicune) from 12th Movie Giveaway unlocks some Zoroark event in B/W.
And, event-key items are really key, first you get them (eg. Oak's Letter, Member Card or Azure Flute) then catch legendaries (Shaymin, Darkrai or Arceus). The same is true for older games, like tickets, maps or keys.