Photo & owned by Shadouge.

Similar theme, different era-design-styles. The left pair has only spinning classic Sonic in line-art & color, while the right has modern Sonic & Tails, however, Tails is never colored/always line art. There are also star bumpers in blue. Both available at New Look in 2011 

A similar checker theme, but different places to find them. The right pair has small red checkers with various scattered classic Sonic faces, & the right is a bit bolder, with large red checkers & 'tilting' classic Sonic with a good big logo. Right pair found at Amazon UK, left at New Look Photo & owned by Shadouge.

 Photo & owned by Shadouge.

This is a classic style pair of boxers which uses "Sonic" written in a blocky font as the background, but also for a somewhat striped look. The stock art here is all the same thing the "Die Pose" for classic Sonic. It's a bit of an unusual choice, and the fact that it appears in a flame shape of yellow/orange is also odd. This pair has red accents and the classic logo for the waist band. This is available at: New Look. 

 Available at Primark in 2011

This is the only pair of boxers found at Primark in the UK. They look rather similar to the others, though you'll notice that only this pair has a blue tag on the waist band. This is classicly styled, with several of the old stock arts. It also has various small starbursts and sound effects written on it in comic style, such as "Bang, Boom, Blong". Sound effects and underwear are generally not the best combination...but the words do add interest for the background. 

 Photo & owned by Shadouge.

This design has been seen before, but on a poster. The different (non logo) font for Sonic (kind of hard to see among the pattern) also appeared elsewhere, along with the small black/gray checkering. Sonic himself is repeated several times in the same pose to help form the pattern. With colors so close in shade, it can be a bit of a confusing design. Found at New Look UK in 2011. 

 Photo & owned by Shadouge.

It's slightly hard to tell here, but this pair of boxers also uses the blocky font "Sonic" to form the stripes. You can see it written in yellow, green, red & blue. Scattered over this are various classic style Sonics, sometimes mirrored. The leg bands for this may also be elastic. Found at New Look in 2011 spring.