Sonic really does look bad-ass as a 3D character, doesn't he? That hint of a smile on his face tells me he's ready to go kick some ass, but not without acknowledging how successful we have helped him become.
I do that on average to look for some references, or to make personal wallpapers. I think I've seen all the ones available too many times to count, I can actually identify which ones are the basis for some of the merchandise.
I want you to actually source me here.
Just because someone can make a convincing character model to create new and interesting poses and action shots does not mean it's automatically stolen. Some people are just very good at 3D modeling and can create something that looks just like the same style as the original art. You just have to have the right dimensions, measurements, and take the time to do it until it's you feel it's right.
Look through the gallery. There's plenty of recreations of 2D artwork, defenders of old ones in a more updated style, reposing a with other characters, and... Well, if you look at the Mephiles artwork, it's in higher quality than what's availible officially (Note: Mephiles the Dark never had official art renders for promotional images, so there's nothing those could be stolen from). There's even full landscapes and full scenes modeled by this person, and they have videos of model swaps they've done with the games themselves, so this person is obviously talented with the computer and 3D modeling.
As a major Sonic fan myself (been so since 1992, when I was 2 years old...), I can tell if official art has been ripped or not pretty well. This seems pretty original and there's not a single official render that it could be stolen from.