In Pokemon Omega Ruby & Alpha Sapphire: Special Demo Version, you get to choose a starter for the initial mission,
evolve your starter, get the Mega Stone for your first Mega Evolution in the Special Demo Version,
defeat Team Magma & Aqua admins, and capture a Glalie International region 3DS / Steelix JP region 3DS, that will be transferred to the main game,
For subsequent missions, you'll begin the demo with the final evolution of all the starters, all holding their Mega Stones.
Accompanying you, is also the Pokemon obtained from the initial mission, that is also holding it's Mega Stone.
This page contains various for these Pokemon, as well as other unobtainable Pokemon encountered throughout this Special Demo Version, obtained by @jojo12100 & @BlackShark.
What we have, but not limited to:
Initial Starters (Grovyle, Combusken, Marshtomp)
Mega Stone Holding Starters (Sceptile, Blaziken, Swampert)
Mega Evolving Gift Pokemon (Glalie & Steelix)
Steven's Shiny Metagross
Team Magma Admin Tabitha's Camerupt
Team Aqua Admin Matt's Sharpedo
Various other Wild Pokemon, including Wild Shroomish (that cannot be caught)
Missing entries that I'm aware of:
Secret Mission: May's Pokemon