These are both t-shirt designs, but they are so detailed & complicated that the whole shirt doesn't fit in there with them / has been cropped away so you get design close-ups. The shirts are just plain black anyway, with no other design element but these, printed large on the fronts. These are both Sega Shop online shirts that appear in 2018, and should be about 20 dollars each or so, in adults sizes only. The designs are both the same off-gold color, and no part is filled in, they are all linework.

The first is "Green Hill Zone Adventurer" with modern running Sonic in proflie. There's Japanese text at the top, but it involves lots of Kanji characters so you can't read it /guess it. Lots of tiny lines make up a loop, rings & 2 sunflowers from GHZ. There are also rays, stars, a hidden Emerald, and speed lines as well as shader lines on Sonic himself.
The second is more Green Hill, complete with polygonal palm trees, another loop, 2 kinds of flowers, uneven ground, rings, and cloud background. Sonic here is the jumping thumbs-up art of Modern Sonic, and above his thumb is a ring and some sparkles.
Both of these shirts are really unique and just packed with detail. No previous shirt uses any sort of similar design, so if you're looking for something very fresh, either design is a good pick.