As
@Trilby pointed out, Sonic himself has been described as having strong elements of Felix the Cat in his design. When that character first became popular, there were loads of rip-offs. All the other studios had to do was draw a black-and-white figure with a huge head and rubbery limbs, change the ears and maybe the nose and yell "ORIGINAL CHARACTER DO NOT STEAL." Oswald the Lucky Rabbit, Bosko, Bimbo, even Mickey Mouse all originated this way (despite Disney's bullshit claims otherwise).
What I'm getting at, via cartoon history sperging, is that it's surprisingly easy to create an "original character" that won't get you into too much trouble. Howard the Duck started out as a visual parody of Donald Duck, but when Disney complained, all Marvel had to do was change his outfit and they were fine.