Yveltal

Yvetal => is Pokemon dex #717 (Pokemon Legendary)

Opendex
=> Pokemon you already have/registered in your Pokedex in the form of images and Pokemon details

Non-Opendex
=> Pokemon you don't have yet/not registered in your Pokedex, appearing as a black shadow (meaning you've seen it in raids/gyms) or just a Pokedex number

Requirements for Trading Opendex:
1. Must have 100 Stardust
2. Must be registered (Opendex) in your Pokedex

Requirements for Trading Non-Opendex:
1. Must have 20,000 Stardust
2. Still a shadow or you don't own the Pokemon yet
(For Yvetal specifically, there's a way to turn Non-Opendex into Opendex—contact the seller after purchase. Valid until June 1, 2021.)

Endex
1. Must have 100 Stardust
2. Still a shadow or you don't own the Pokemon yet

NB=> Tip for Non-Opendex (Legendary Pokemon/Shiny Pokemon): Be willing to build friendship up to Ultra Friend level to reduce Stardust cost (80,000 Stardust). If interested, buy the cheapest Opendex Pokemon or one you like from Wantau Store, then chat with the seller to request a 30-day friendship (Ultra Friend).

How to Trade Opendex or Non-Opendex? Check the link below: