How to remove background from a Pokédex/in-game video capture (after the video was turned into a gif):
https://projectpokemon.org/home/forums/topic/43730-help-removing-background-of-3ds-sprites-are-missing/?do=findComment&comment=228819

You may have to cycle through the "white background" process a few times to remove all the blips and leftover dots from the background.

If the model itself has colors removed, you may have to reduce the fuzziness, and go through the process multiple times. If the model is still affected, you may need to take the clean gif, and use Split: Gif to sprite sheet and add back the affected colors in. (For example, I did that with Zekrom's tail above)