The rectangle looks about the size of the old "watch next" icons that creators used to be able to put on end cards.
If I had to guess, the stegosaurus back was always like that but it was covered up by another video thumbnail, or when YouTube removed the feature they filled in the space by copying whatever was on left.
Wouldn't make sense as the thumbnail though?
Disclaimer: I've never uploaded anything to YouTube, so this is just speculation.
I think there used to be the "old" video editing tools that let creators choose an end-card and previews thumbnails etc. I believe they changed the tooling a few times, and I could imagine a scenario where the preview thumbnail was missing or defaulted and the migration just used some other available still from the video...
The alternative theory is that CGP grey had an aneurysm while making the preview thumbnail and nobody has noticed except you for 7 years.
Seen that stuff a few month back.
I think it a problem with A/B testing thumbnails, because sometimes the chunk is clearly from an entirely different image that also fits the video.
