● The Nike logo, Protoman and Japanede folklore
Knuckles has a white birthmark across his chest, which was inspired by the Nike swoosh symbol. He was originally supposed to be green, maybe because Knuckles was meant to be Jamaican and speak with a Jamaican accend. In the end the Jamaican character idea was cut and Knuckles becames red. This change could have some reference from Protoman from Megaman series.
Protoman has a Story like Knuckles', because they both were enemies and appeared in the hero's way tying to stop him. Besides Protoman isn't a enemy anymore and help Megaman sometimes, his red colour contrasts to the hero's blue colour, just like happens to Knuckles and Sonic.
The idea of a two tailed fox used in Tails' design process have resemblances with Kitsune, a fox of Japanese folklore.
According to the folktales, these foxes are believed to posses superior intelligence, long life and magical powers. The more tails a Kitsune has, the older, wiser and more powerful it is and they may have as many as nine tails.
Talking about resemblances, it's possible that Sonic's design could be based on the most famous mouse in the world: Mickey Mouse.
Sega rathered the classic Mickey's style, without a shirt, wearing red shorts and yellow shoes, when they made Castle Of Illusion, One of the firsts Sega Genesis games and a great hit.
At this time, Sega needed to create a new character to be their mascot and wich could be more popular than Mario from Nintendo, and wich could represents Sega like Mickey represents Disney.
Among the new ideas, the better was a rabbit. His concept design has very resemblance to Oswald The Lucky Rabbit, the character Walt Disney usually drew before the Mickey creation. Sega was going to make the new rabbit's game just like the Master System version of Castle Of Illusion, where you catch objects in the stages and throw against the enemies. Then Yuji Naka cames with another idea for the gameplay and Naoto Ohshima did the firsts and the final designs of Sonic The Hedgehog wich was released in 1991. There's a lot of resemblances between the Sonic's and the Mickey's design, specially when compared with the rabbit wich was the first Sega's idea.
The body proportion in relation to the head, the thin legs and arms, the eyes and the eyebrows, the hands wearing gloves, all these details have resemblance between them.
Another interesting resemblance is noticed in Castle Of Illusion, because Mickey also have to collect the 7 powerful Rainbow Gems defeating the enemies.