So as we've seen from Sonic Adventure 1 and Sonic Adventure 2, Knuckles had some rap themes to give off the character's personality in said games. Jokingly, because of this and because of some of the nature of the music, the popular assumption is that he himself is the "black" character in Sonic games similar to the joke that Piccolo is in DBZ. (Dred Foxx did the rapping in the original Knuckles theme, Unknown from M.E. in Sonic Adventure 1 and Hunnid P did in Sonic Adventure 2)
With that said, he's actually Mayan, or based upon that.
You can see this in Sonic Adventure where on the Floating Island there are remains of very similar to Mayan architecture, and later on in the past you see that the Knuckles Clan live in Mayan themed temples and their artwork is similar to that of the Mayans.
There's also the tribal markings the Knuckles Clan feature.
Tikal, one of the main characters and part of the Knuckles Clan's name is actually based off an area that has a Mayan temple.
http://en.wikipedia.org/wiki/Tikal
So there you have it!
With that said, he's actually Mayan, or based upon that.
You can see this in Sonic Adventure where on the Floating Island there are remains of very similar to Mayan architecture, and later on in the past you see that the Knuckles Clan live in Mayan themed temples and their artwork is similar to that of the Mayans.
There's also the tribal markings the Knuckles Clan feature.
Tikal, one of the main characters and part of the Knuckles Clan's name is actually based off an area that has a Mayan temple.
http://en.wikipedia.org/wiki/Tikal
So there you have it!